Offender is soldier perhaps behind nasty crimes (9)
I believe the answer is:
miscreant
'offender' is the definition.
(miscreant is a kind of offender)
'soldier perhaps behind nasty crimes' is the wordplay.
'soldier perhaps' becomes 'ant' (soldier is a type of ant).
'behind' is a charade indicator (letters next to each other).
'nasty' is an anagram indicator.
'crimes' with letters rearranged gives 'miscre'.
'ant' after 'miscre' is 'MISCREANT'.
'is' acts as a link.
(Other definitions for miscreant that I've seen before include "Offender" , "villain" , "Naughty boy!" , "Wretch" , "culprit" .)