Very old diamonds, say (5)
I believe the answer is:
voice
'say' is the definition.
(voicing is a kind of saying)
'very old diamonds' is the wordplay.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'diamonds' becomes 'ice' (I've seen this before).
'v'+'o'+'ice'='VOICE'
(Other definitions for voice that I've seen before include "tenor, say" , "Speak of - mouthpiece" , "Put into words" , "Wallop" , "Opinion; part in a fugue" .)