Opposite an American theatre award on top of mantelpiece (7)
I believe the answer is:
antonym
'opposite' is the definition.
('antonym' can be a synonym of 'opposite')
'an american theatre award on top of mantelpiece' is the wordplay.
'an american theatre award' becomes 'antony' (I've seen this in other clues).
'on top of' says to take the initial letters (I've seen 'on top' mean this).
The initial letter of 'mantelpiece' is 'm'.
'antony'+'m'='ANTONYM'
(Other definitions for antonym that I've seen before include "something completely different" , "A word opposite in meaning to another" , "opposing description" , "As 'happy' is to 'sad' or 'long' to 'short'" , "as host is for guest, eg" .)