Buyer's limit included in letter (6)
I believe the answer is:
vendee
'buyer's' is the definition.
(synonyms)
'limit included in letter' is the wordplay.
'limit' becomes 'end' (I've seen this before).
'included in' means one lot of letters goes inside another.
'letter' becomes 'vee' (I've seen this in another clue).
'end' placed inside 'vee' is 'VENDEE'.
(Other definitions for vendee that I've seen before include "Region in west central France, home to Challons" , "Department of France on the Bay of Biscay" , "Buyer; area of Western FRance" , "Evened out the area in France on the Atlantic" , "Coastal department of western France" .)