Piece of asparagus with perfect flavour (9)
I believe the answer is:
spearmint
'flavour' is the definition.
(spearmint is a flavour)
'piece of asparagus with perfect' is the wordplay.
'piece of asparagus' becomes 'spear' (I've seen this in another clue).
'with' is a charade indicator (letters next to each other).
'perfect' becomes 'mint' (similar in meaning).
'spear'+'mint'='SPEARMINT'
(Other definitions for spearmint that I've seen before include "Garden flavouring" , "fresh flavour" , "Herb used for flavouring" , "Aromatic plant" , "Strongly flavoured herb" .)