Two mats over a big gong (3-3)
I believe the answer is:
tam-tam
'a big gong' is the definition.
(I've seen this in another clue)
'two mats over' is the wordplay.
'two mats' can mean two replacements for 'mat'.
'over' says the letters should be written in reverse.
'mat'+'mat'='matmat'
'matmat' written backwards gives 'TAM-TAM'.
(Other definitions for tam-tam that I've seen before include "Large musical gong" , "Large metal gong" , "Large orch. gong" , "Percussion instrument" .)