Opening bid's heart, for playing before diamonds (7)
I believe the answer is:
orifice
'opening' is the definition.
('orifice' can be a synonym of 'opening')
'bid's heart for playing before diamonds' is the wordplay.
'heart' indicates the central letters.
'playing' indicates anagramming the letters.
'before' is a charade indicator (letters next to each other).
'diamonds' becomes 'ice' (I've seen this before).
The middle letter of 'bid' is 'i'.
'i'+'for'='ifor'
'ifor' anagrammed gives 'orif'.
'orif'+'ice'='ORIFICE'
(Other definitions for orifice that I've seen before include "Space" , "Vent" , "Hole, opening" , "Hole in the head, perhaps?" , "Mouth-like opening" .)