Artist gatecrashes every exhibition, principally as a complaint (7)
I believe the answer is:
earache
'a complaint' is the definition.
(a type of medical complaint)
'artist gatecrashes every exhibition principally' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'gatecrashes' is an insertion indicator (inserted letters gatecrash their way into the word).
'every' becomes 'each' (synonyms).
'principally' suggests taking the first letters.
The first letter of 'exhibition' is 'e'.
'ra' inserted inside 'each' is 'earach'.
'earach'+'e'='EARACHE'
'as' acts as a link.
(Other definitions for earache that I've seen before include "Pain in aural organ" , "Each era (anag)" , "Otic pain" , "otitis" , "minor complaint" .)