Soldiers in prison show remorse (6)
I believe the answer is:
repent
'show remorse' is the definition.
(I've seen this in another clue)
'soldiers in prison' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'in prison' becomes 'pent' ('pent' can mean 'confined').
're'+'pent'='REPENT'
(Other definitions for repent that I've seen before include "Turn away from sin" , "Be sorry for wrongdoing" , "Feel sincere regret" , "Be contrite, ye sinners" , "Show remorse" .)