Improve behaviour of soldiers with criminal record (6)
I believe the answer is:
reform
'improve behaviour of' is the definition.
'soldiers with criminal record' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'with' is a charade indicator (letters next to each other).
'criminal record' becomes 'form' (I've seen this in other clues**).
're'+'form'='REFORM'
(Other definitions for reform that I've seen before include "Correct(ion of) abuses" , "Mend one's ways" , "Removal of abuses" , "Change one's ways (for the better?)" , "Go straight" .)