Gibe artist admitted to ruins a festival (5,4)
I believe the answer is:
mardi gras
'a festival' is the definition.
(Mardi Gras is an example)
'gibe artist admitted to ruins' is the wordplay.
'gibe' becomes 'dig' (synonyms).
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'admitted to' means one lot of letters goes inside another.
'ruins' becomes 'mars' (to mar is to ruin or spoil).
'dig'+'ra'='digra'
'digra' going within 'mars' is 'MARDI GRAS'.
(Other definitions for mardi gras that I've seen before include "pre-Lent festival" , "festive occasion" , "Day for 9 acrosses" , "Revelry celebrating Shrove Tuesday" , "Shrove Tuesday festival" .)