noun, noun or participle which takes 'suru'
returning to a club; rejoining a club
Colloquial and rare; used for rejoining a club or circle one previously belonged to. Often used with する.
大学のサークルに復部することにした。
I decided to rejoin my university club.
復帰 is a general term for returning or reinstatement, while 復部 is specifically about rejoining a club or circle.
Compound of 復 (return) and 部 (club). The exact derivation is uncertain; it appears to be a modern colloquial coinage.