Top-notch crust of rhubarb tart (5)
I believe the answer is:
acerb
'tart' is the definition.
(I've seen this in another clue)
'top-notch crust of rhubarb' is the wordplay.
'top-notch' becomes 'ace' (ace can informally mean very good).
'crust of' suggests removing the centre (the external letters or crust of the word).
'rhubarb' with its middle taken out is 'rb'.
'ace'+'rb'='ACERB'
(Other definitions for acerb that I've seen before include "tart" , "harsh and bitter" , "Sour, bitter" , "sharp" .)