Be sorry for soldiers cooped up (6)
I believe the answer is:
repent
'be sorry for' is the definition.
(I've seen this before)
'soldiers cooped up' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'cooped up' becomes 'pent' (both can mean enclosed).
're'+'pent'='REPENT'
(Other definitions for repent that I've seen before include "Feel or express contrition" , "Feel guilty" , "Turn away from sin" , "Feel sorry" , "Be ashamed" .)