But Diana Ross would have no use for it! (5)
I believe the answer is:
dross
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'but diana ross would have no use' is the wordplay.
I cannot really see how this works, but
'would' could be 'd' (as in 'we'd' - 'we would') and 'd' is present in the answer.
'no' could be 'o' (resembles 0 - 'no' can mean 'zero') and 'o' is found within the answer.
The remaining letters 'rss' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'for' is the link.
Can you help me to learn more?
(Other definitions for dross that I've seen before include "Rubbish, scum" , "Scum on molten metals" , "Worthless or dangerous material" , "Refuse - dregs" , "Anything worthless" .)