Met deride criminal taking on new firm (10)
I believe the answer is:
determined
'firm' is the definition.
(I've seen this before)
'met deride criminal taking on new' is the wordplay.
'criminal' is an anagram indicator.
'taking on' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'met'+'deride'='metderide'
'metderide' anagrammed gives 'determied'.
'determied' placed around 'n' is 'DETERMINED'.
(Other definitions for determined that I've seen before include "Found out" , "Decided - resolute" , "Resolved" , "set" , "firm" .)