senior group member; leader among peers
Refers to a person who acts as an older-brother figure or senior within a group, especially in informal or hierarchical settings like gangs, clubs, or workplaces.
He is relied on by everyone as the big-brother figure of the team.
like an older brother; big brother
Used in the pattern 「〜の兄貴分」 to describe someone who is like an older brother to another person. The cross-reference to 分 (ぶん) sense 4 indicates this is a suffix-like usage meaning 'in the role of'.
See also: 分 (ぶん)
Mr. Tanaka is like an older brother to me.
Compound of 兄貴 (あにき, 'older brother' or 'senior') and 分 (ぶん, 'portion' or 'role'). The 分 here indicates a share or role, giving the meaning 'one who takes the role of an older brother'.