Complain after ancient coin found in rhubarb dish (5,4)
I believe the answer is:
roast beef
'dish' is the definition.
(I've seen this before)
'complain after ancient coin found in rhubarb' is the wordplay.
'complain' becomes 'beef' (beefing is a kind of complaining).
'after' says to put letters next to each other.
'ancient coin' becomes 'as' (ancient Roman coin).
'found in' means one lot of letters goes inside another.
'rhubarb' becomes 'rot' (I've seen this before).
'as' inserted within 'rot' is 'roast'.
'beef' after 'roast' is 'ROAST-BEEF'.
(Other definitions for roast beef that I've seen before include "Typical English dish" , "Joint" , "traditional meal" , "part of lunch for Sun?" , "meat dish" .)