Shatter grail after silly poem (8)
I believe the answer is:
madrigal
'poem' is the definition.
(thesaurus)
'shatter grail after silly' is the wordplay.
'shatter' indicates anagramming the letters (I've seen this in other clues).
'after' is a charade indicator (letters next to each other).
'silly' becomes 'mad'.
'grail' with letters rearranged gives 'rigal'.
'rigal' after 'mad' is 'MADRIGAL'.
(Other definitions for madrigal that I've seen before include "Elaborate part song" , "Drag Liam around to hear an unaccompanied song" , "Without exaggeration" , "Elizabethan song" , "glee" .)