Upset girls hiding at German camp full of belligerent men (6)
I believe the answer is:
stalag
'german camp full of belligerent men' is the definition.
(German prisoner of war camp)
'upset girls hiding at' is the wordplay.
'upset' is a reversal indicator.
'girls' becomes 'gals' ('gal' can be a synonym of 'girl').
'hiding' is an insertion indicator.
'gals' enclosing 'at' is 'galats'.
'galats' reversed gives 'STALAG'.
(Other definitions for stalag that I've seen before include "German prison camp in WW2" , "German concentration camp" , "German prison_camp" , "German POW camp in WWII" , "Water at the mouth" .)