One serving in trade receives tip (9)
I believe the answer is:
bartender
'one serving' is the definition.
(I've seen this before)
'trade receives tip' is the wordplay.
'trade' becomes 'barter' (synonyms).
'receives' means one lot of letters goes inside another (inserted letters are received).
'tip' becomes 'end' (tip can mean the end of something).
'barter' enclosing 'end' is 'BARTENDER'.
'in' is the link.
(Other definitions for bartender that I've seen before include "Liquor salesman" , "Pub employee" , "Someone serving drinks" , "American serving drinks" , "Taverner" .)