Outlaw finally joined gang (4)
I believe the answer is:
band
'gang' is the definition.
(I've seen this before)
'outlaw finally joined' is the wordplay.
'outlaw' becomes 'ban' (banning is a kind of outlawing).
'finally' says to take the final letters.
The last letter of 'joined' is 'd'.
'ban'+'d'='BAND'
(Other definitions for band that I've seen before include "Players" , "Zone; group" , "Pop group; stripe" , "Group; frequency range" , "Belt; group of musicians" .)