Raise the price after agreeing a lower one (6)
I believe the answer is:
gazump
'raise the price after agreeing a lower one' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gazump that I've seen before include "Raise a house price after offer accepted" , "Successfully outbid someone with already accepted offer on a house" , "Offer more for a house than (an earlier bidder)" , "Make late higher bid for house" , "Raise the house price after accepting an offer" .)