Supply a hardback at a price (4)
I believe the answer is:
feed
'supply' is the definition.
(feeding is a kind of supplying)
'a hardback at a price' is the wordplay.
'a hardback' becomes 'd' (I can't explain this - if you can you should believe this answer much more).
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'a price' becomes 'fee' (I've seen this before).
'd' after 'fee' is 'FEED'.
Can you help me to learn more?
(Other definitions for feed that I've seen before include "forage" , "take grub" , "Give (or take) nourishment" , "Straight man" , "Given payment" .)