Old coin found in tart, not the last (3)
I believe the answer is:
sou
'old coin' is the definition.
(historical French coin)
'tart not the last' is the wordplay.
'tart' becomes 'sour' (similar in meaning).
'not the last' means to remove the last letter.
'sour' with its final letter taken away is 'SOU'.
'found in' is the link.
(Other definitions for sou that I've seen before include "Former small French coin" , "Old French monetary unit" , "Old French coin of little value" , "bit of money" , "16 French coin" .)