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 quite see how this works, but
'would' could be 'd' (as in 'we'd' - 'we would') and 'd' is located in the answer.
'no' could be 'o' (O resembles 0) and 'o' is found in 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' acts as a 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" .)