Fruit mostly smelt awful when brought up (4)
I believe the answer is:
nuts
'fruit' is the definition.
(I know that nuts is a type of fruit)
'mostly smelt awful when brought up' is the wordplay.
'mostly' means to remove the last letter.
'smelt awful' becomes 'stunk'.
'when brought up' shows that the letters should be reversed in order.
'stunk' with its final letter removed is 'stun'.
'stun' reversed gives 'NUTS'.
(Other definitions for nuts that I've seen before include "Eg, filberts, hazels" , "Edible seeds with hard shells" , "Delivers 24 [HEADBUTT]" , "Mad; seeds" , "Mad; fruit" .)