Marine finally joining crack outfit (5)
I believe the answer is:
equip
'outfit' is the definition.
('equip' can be a synonym of 'outfit')
'marine finally joining crack' is the wordplay.
'finally' says to take the final letters.
'joining' says to put letters next to each other.
'crack' becomes 'quip' ('quip' can be a synonym of 'crack'**).
The final letter of 'marine' is 'e'.
'e'+'quip'='EQUIP'
(Other definitions for equip that I've seen before include "Outfit" , "Give everything needed" , "Give what's needed" , "Provide with items (for particular task)" , "Furnish with requirements" .)