One of fifty diamonds, it's said (6)
I believe the answer is:
stated
'said' is the definition.
('state' can be a synonym of 'say')
'one of fifty diamonds' is the wordplay.
'one of fifty' becomes 'state' (I've seen this in another clue).
'diamonds' becomes 'd' (abbreviation in bridge say).
'state'+'d'='STATED'
'it's' acts as a link.
(Other definitions for stated that I've seen before include "Said firmly, averred" , "Definitely expressed" , "Set forth (in words)" , "Uttered, expressed in words" , "Put in words, said" .)