Advice to sinners and soldiers imprisoned (6)
I believe the answer is:
repent
'advice to sinners' is the definition.
(I've seen this in another clue)
'soldiers imprisoned' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'imprisoned' becomes 'pent' (obsolete past tense form of 'pen').
're'+'pent'='REPENT'
'and' acts as a link.
(Other definitions for repent that I've seen before include "Feel or express contrition" , "Show remorse" , "Be sorry for - . . . . . . ye sinners" , "Be ashamed" , "John the Baptist's cry" .)