Surface belly up? (3)
I believe the answer is:
top
'surface' is the definition.
(top is a kind of surface)
'belly up?' is the wordplay.
'belly' becomes 'pot' ('pot' can be a synonym of 'belly').
'up?' shows that the letters should be reversed in order (in down clue: letters go upwards).
'pot' in reverse letter order is 'TOP'.
(Other definitions for top that I've seen before include "Better" , "premier" , "Peak; child's toy" , "The upper part" , "Crown" .)