Shady soldiers let in subject to debate (12)
I believe the answer is:
disreputable
'shady' is the definition.
(similar in meaning)
'soldiers let in subject to debate' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'let in' is an insertion indicator.
'subject to debate' becomes 'disputable' (able to be disputed).
're' put into 'disputable' is 'DISREPUTABLE'.
(Other definitions for disreputable that I've seen before include "Infamous - shabby" , "Not respectable, shabby" , "Not of decent character" , "not proper or decent!" , "Superb detail (anag) -shady" .)