Month involved poet in fraud (9)
I believe the answer is:
deception
'fraud' is the definition.
(I've seen this before)
'month involved poet in' is the wordplay.
'month' becomes 'dec' (abbreviation for December).
'involved' indicates an anagram (involved can mean confusing or complex).
'poet'+'in'='poetin'
'poetin' is an anagram of 'eption'.
'dec'+'eption'='DECEPTION'
(Other definitions for deception that I've seen before include "Fraud" , "Duplicity" , "Illusion" , "Trick" .)