Mosaic piece's actual existence in art brought up (7)
I believe the answer is:
tessera
'mosaic piece's' is the definition.
(I've seen this in another clue)
'actual existence in art brought up' is the wordplay.
'actual existence' becomes 'esse' (I've seen this before).
'in' means one lot of letters goes inside another.
'brought up' is a reversal indicator (in a down clue, letters go up).
'esse' placed within 'art' is 'aresset'.
'aresset' in reverse letter order is 'TESSERA'.
(Other definitions for tessera that I've seen before include "Bit of mosaic" , "Piece of picture" , "Small mosaic piece" , "Die" , "Reseats (anag) -- small tile used in a mosaic" .)