Excellent entertainment, not entirely suitable for solemn period (6)
I believe the answer is:
lenten
'suitable for solemn period' is the definition.
(suitable for Lent)
'excellent entertainment not entirely' is the wordplay.
'not entirely' indicates a hidden word.
'LENTEN' is hidden in the letters of 'excellent entertainment'.
(Other definitions for lenten that I've seen before include "Of the time just before Easter" , "Of the pre-Easter period" , "Refers to the period before Easter" , "Pertaining to time of fast" , "Of the Christian penitential season" .)