Trooper occupying the back makes up crew (5)
I believe the answer is:
eight
'crew' is the definition.
(an eight-person rowing team)
'trooper occupying the back' is the wordplay.
'trooper' becomes 'gi' (term for US soldier).
'occupying' means one lot of letters goes inside another.
'back' shows that the letters should be reversed in order.
'gi' put inside 'the' is 'thgie'.
'thgie' written backwards gives 'EIGHT'.
'makes up' is the link.
(Other definitions for eight that I've seen before include "No. of stones in cwt." , "Team involved in row" , "Half a quarter" , "Two cubed" , "Perfect cube" .)