Lettuce at back-street prices (5)
I believe the answer is:
costs
'prices' is the definition.
('cost' can be a synonym of 'price')
'lettuce at back-street' is the wordplay.
'lettuce' becomes 'cos' (cos is a kind of lettuce).
'at' is a charade indicator (letters next to each other).
'back' says the letters should be written backwards.
'street' becomes 'st' (abbreviation).
'st' in reverse letter order is 'ts'.
'cos'+'ts'='COSTS'
(Other definitions for costs that I've seen before include "prices" , "Is priced at" , "Charges" , "is expensive" , "Damages" .)