Compromising one's art a capital offence? (7)
I believe the answer is:
treason
'a capital offence?' is the definition.
'compromising one's art' is the wordplay.
'compromising' indicates anagramming the letters (compromise can mean change).
'ones'+'art'='onesart'
'onesart' is an anagram of 'TREASON'.
(Other definitions for treason that I've seen before include "One's art? (anag)" , "Perfidy" , "subversion" , "Crime of treachery against the state" , "Lese-majesty" .)