Noel possibly involved in extremely fishy crime (6)
I believe the answer is:
felony
'crime' is the definition.
(felony is a kind of crime)
'noel possibly involved in extremely fishy' is the wordplay.
'possibly' is an anagram indicator.
'involved in' is an insertion indicator.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'fishy' with its middle removed is 'fy'.
'noel' anagrammed gives 'elon'.
'elon' going within 'fy' is 'FELONY'.
(Other definitions for felony that I've seen before include "Criminal offence" , "Criminal act" , "Serious crime" , "Grave crime" .)