Art brought up on unknown platter (4)
I believe the answer is:
tray
'platter' is the definition.
(I've seen this before)
'art brought up on unknown' is the wordplay.
'brought up' is a reversal indicator (down clues only).
'on' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
'art' backwards is 'tra'.
'tra'+'y'='TRAY'
(Other definitions for tray that I've seen before include "canteen item" , "Shallow receptable for carrying something" , "Correspondence container" , "Item used for carrying food and drink" , "Domestic carrier" .)