Soldiers imprisoned feel remorse (6)
I believe the answer is:
repent
'feel remorse' is the definition.
(I've seen this before)
'soldiers imprisoned' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'imprisoned' becomes 'pent' (obsolete past tense form of 'pen').
're'+'pent'='REPENT'
(Other definitions for repent that I've seen before include "Be contrite, ye sinners" , "Deplore" , "Be ashamed" , "Feel remorse for" , "Feel sorry" .)