A criminal securing crooked mate's remission (9)
I believe the answer is:
abatement
'remission' is the definition.
(thesaurus)
'a criminal securing crooked mate's' is the wordplay.
'criminal' becomes 'bent' (I've seen this before**).
'securing' indicates putting letters inside.
'crooked' indicates an anagram.
'mate' is an anagram of 'atem'.
'bent' placed around 'atem' is 'batement'.
'a'+'batement'='ABATEMENT'
(Other definitions for abatement that I've seen before include "Subsidence" , "relief" , "Lessening in intensity or amount" , "mitigation" .)