Senator grins wickedly about statesman's first offence (13)
I believe the answer is:
transgression
'offence' is the definition.
(thesaurus)
'senator grins wickedly about statesman's first' is the wordplay.
'wickedly' indicates anagramming the letters.
'about' indicates putting letters inside.
'first' indicates taking the first letters.
The initial letter of 'statesmans' is 's'.
'senator'+'grins'='senatorgrins'
'senatorgrins' anagrammed gives 'trangression'.
'trangression' going around 's' is 'TRANSGRESSION'.
(Other definitions for transgression that I've seen before include "Wrongdoing" , "Wrong" , "Infringement; sin" , "something bad going on?" , "Offence" .)