Set rate paid, inclusive of grain (6)
I believe the answer is:
priced
'set rate' is the definition.
(to price something is to set its price or rate)
'paid inclusive of grain' is the wordplay.
'paid' becomes 'pd' (I've seen this in other clues).
'inclusive of' means one lot of letters goes inside another.
'grain' becomes 'rice' (rice is a kind of grain).
'pd' placed around 'rice' is 'PRICED'.
(Other definitions for priced that I've seen before include "Valued for sale" , "Assigned value for sale" , "Maybe labelled with amount to be paid" , "Costed" .)