Men have fun holding nothing back (6)
I believe the answer is:
troops
'men' is the definition.
(I've seen this before)
'have fun holding nothing back' is the wordplay.
'have fun' becomes 'sport'.
'holding' is an insertion indicator.
'nothing' becomes 'O' (looks like zero - 0).
'back' is a reversal indicator.
'sport' enclosing 'o' is 'spoort'.
'spoort' reversed gives 'TROOPS'.
(Other definitions for troops that I've seen before include "Cavalry" , "men" , "Groups of soldiers" , "Soldiers, armed forces" , "Army" .)