18 across
Coming
back
discontented,
negotiates
very
low
price,
a
bargain
(4)
Coming back discontented, negotiates very low price, a bargain (4)
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The definition is at the end of the clue.
How the clue works
The answer is a word for a bargain. Take "negotiates" and remove its middle (discontented), leaving the outer letters NS, then reverse them (coming back) to get SN; follow that with IP, standing for one penny, a very low price.