What's good in a crumble? Nonsense! (7)
I believe the answer is:
rhubarb
I believe this clue is a double definition.
'what's good in a crumble?' is the first definition.
(rhubarb crumble is a type of crumble)
'nonsense' is the second definition.
(rhubarb can mean nonsense or rubbish)
(Other definitions for rhubarb that I've seen before include "Reddish leafstalks cooked and eaten as fruit" , "Garden plants with fleshy stalks or slang nonsense" , "making contribution to murmuring" , "perennial plant" , "Sour sticks eaten as fruit after cooking" .)