Understand artist interrupting planet's festival (5,4)
I believe the answer is:
mardi gras
'festival' is the definition.
(Mardi Gras is an example)
'understand artist interrupting planet's' is the wordplay.
'understand' becomes 'dig' (digging is a kind of understanding).
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'interrupting' indicates putting letters inside.
'planet' becomes 'mars' (Mars is a planet).
'dig'+'ra'='digra'
'digra' placed within 'mars' is 'MARDI GRAS'.
(Other definitions for mardi gras that I've seen before include "Shrove Tuesday carnival" , "Carnival (day)" , "Revelry celebrating Shrove Tuesday" , "The lead up to fast" , "pre-Lent festival" .)