Bargain: got one in smart sale at the finish (9)
I believe the answer is:
'bargain' is the definition.
(I know that bargain is a more specific form of the action negotiate)
'got one in smart sale at the finish' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' means one lot of letters goes inside another.
'smart' becomes 'neat' (synonyms).
'at the finish' indicates one should take the final letters.
The final letter of 'sale' is 'e'.
'goti' placed inside 'neat' is 'negotiat'.
(Other definitions for negotiate that I've seen before include "Pass round" , "Find a way over (a difficult path)" , "Try to reach agreement through discussion" , "as diplomat does" , "I get a note (anag) - bargain" .)