Price paid for lettuce and last of beetroot (4)
I believe the answer is:
'price paid' is the definition.
(I've seen this in another clue)
'lettuce and last of beetroot' is the wordplay.
'lettuce' becomes 'cos' (cos is a kind of lettuce).
'and' is a charade indicator (letters next to each other).
'last of' indicates one should take the final letters.
The final letter of 'beetroot' is 't'.
'for' is the link.
(Other definitions for cost that I've seen before include "Outlay required to buy" , "estimate price" , "Total spent for goods or services" , "penalty" , "Amount to pay" .)