Relief for troops taking part in struggle (6)
I believe the answer is:
effort
'struggle' is the definition.
(I know that struggle can be written as effort)
'relief for troops taking part' is the wordplay.
'taking part' indicates a hidden word.
'EFFORT' is hidden within 'relief for troops'.
'in' acts as a link.
(Other definitions for effort that I've seen before include "Vigorous attempt" , "Strenuous exertion" , "Struggle" , "Endeavour, elbow grease" , "Exertion, elbow grease" .)