Porter worried about favourite artist being the culprit (11)
I believe the answer is:
perpetrator
'the culprit' is the definition.
('perpetrator' can be a synonym of 'culprit')
'porter worried about favourite artist' is the wordplay.
'worried' indicates an anagram (worry can mean to interfere with or disturb).
'about' means one lot of letters goes inside another.
'favourite' becomes 'pet' (synonyms).
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'porter' with letters rearranged gives 'pertor'.
'pet'+'ra'='petra'
'pertor' going around 'petra' is 'PERPETRATOR'.
'being' acts as a link.
(Other definitions for perpetrator that I've seen before include "wrongdoer" , "One committing (an offence)" , "One carrying out (crime)" , "Committer (of a crime)" , "Culprit" .)