Impulsive, gobbling small girl’s vegetable (6)
I believe the answer is:
radish
'vegetable' is the definition.
(radish is a type of vegetable)
'impulsive gobbling small girl's' is the wordplay.
'impulsive' becomes 'rash' (I've seen this before).
'gobbling' is an insertion indicator.
'small girl' becomes 'di' (short for Diana).
'rash' going around 'di' is 'RADISH'.
(Other definitions for radish that I've seen before include "Sir had this vegetable" , "Hottish salad plant" , "Pungent red salad root" , "Pungent, red salad vegetable" , "Edible small root" .)