Paltry sum enthusiast invested in vegetables (7)
I believe the answer is:
peanuts
'paltry sum' is the definition.
(I've seen this before)
'enthusiast invested in vegetables' is the wordplay.
'enthusiast' becomes 'nut' (nut can mean an enthusiast or fan).
'invested in' is an insertion indicator.
'vegetables' becomes 'peas' (peas are a type of vegetable).
'nut' placed within 'peas' is 'PEANUTS'.
(Other definitions for peanuts that I've seen before include "A derisory amount of money" , "bar snack" , "Very small amount of money" , "pittance" , "Not much money - from low-growing plant" .)