Pirate's gear adopted by gang (7)
I believe the answer is:
brigand
'pirate's' is the definition.
(brigand is a type of thief)
'gear adopted by gang' is the wordplay.
'gear' becomes 'rig' (rig is a kind of gear).
'adopted by' means one lot of letters goes inside another (I've seen this in other clues).
'gang' becomes 'band' (both can mean a group of people).
'rig' inserted into 'band' is 'BRIGAND'.
(Other definitions for brigand that I've seen before include "Robber in a gang" , "Pirate" , "Robber in wild places" , "Robber - darn big (anag)" , "desperado" .)