Individual weight of one diamond, perhaps (9)
I believe the answer is:
singleton
'one diamond perhaps' is the definition.
(bridge term for the only card from its suit)
'individual weight' is the wordplay.
'individual' becomes 'single' ().
'weight' becomes 'ton' (unit of weight).
'single'+'ton'='SINGLETON'
'of' acts as a link.
(Other definitions for singleton that I've seen before include "could be ace on its own?" , "One alone" , "One club maybe" , "spinster or bachelor?" , "one diamond, perhaps" .)