main chance
/ˈmeɪn ˌtʃɑːns/ /ˈmeɪn ˌtʃans/noun
Something which is of principal importance in life; a livelihood; (now) especially the opportunity of enriching or otherwise benefiting oneself. Often in to have an eye to (also for) the main chance: to have consideration for one's own interests; also to be careful of the main chance.
Origin
Late 16th century (in an earlier sense). From main + chance, perhaps sometimes influenced by main.