Greengrocer may give terrier four treats, or not (9)
I believe the answer is:
fruiterer
'greengrocer' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for fruiterer that I've seen before include "Greengrocer" , "Dealer in produce" , "Dealer in apples and pears?" .)