less important than something else► secondary less important than the main subject, problem etc: · The government sees unemployment as a secondary issue.· The study found that women were often reduced to secondary roles in the workplace.secondary to: · Tourism is secondary to oil revenues as a source of income.
► subsidiary formal a subsidiary idea, question, subject etc is less important than the main one but it is connected with it: · If you take the English literature course, you can do linguistics as a subsidiary subject.subsidiary to: · The formulation of a lasting peace settlement was the main objective, and everything else was seen as subsidiary to it.
► incidental happening or existing in connection with or as a result of something else that is more important: · The Red Cross will provide money for food, housing, and incidental expenses.incidental to: · The puzzles are fun, but are incidental to the plot of the book.purely incidental: · The concert is just for fun, really. Any profit we make from it will be purely incidental.
► marginal too small and unimportant to have any useful or noticeable effect: · The difference between the two cars is marginal.· a marginal increase in sales
► peripheral relating to the main activity, question, or subject, but much less important than it: · It is a society in which women's rights and concerns are still treated as peripheral.· Too much money is being spent on peripheral programs when our kids can't read or do basic math.peripheral to: · The romance was peripheral to the movie's main plot.
► be of secondary/minor/less etc importance to be not very important compared to other things: · It's the perfect job for him - the salary is of secondary importance.· Police are increasingly regarding cannabis use as being of minor importance compared with other forms of crime.· We must stop treating mental illness as being of less importance than physical disabilities.
► side issue a subject or question that results from or is connected with the main one being considered, but is much less important: · The meeting seemed to focus on side issues, without ever really addressing the main point.· One of the side issues that arose was what to do with the old equipment when the new things arrived.
► pale into insignificance if someone's problems or achievements pale into insignificance when they are compared to something else, they are clearly much less important: · Our difficulties pale into insignificance when compared to the problems of the homeless.
a subject or problem that is not as important as the main one, and may take people’s attention away from the main subject: 
The tax proposal is really a side issue with us.