Crossword Genius

A piece of questionable length? (6)

Ross

I believe the answer is:

string

I'm a little stuck... Click here to teach me more about this clue!

'questionable length?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'a piece' is the wordplay.
I cannot really understand how this works, but
'piece' could be 'ring' (I have seen 'Piece of jewellery ' mean 'ring' so perhaps 'piece' could also mean 'ring') and 'ring' is located in the answer.
The remaining letters 'st' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'of' acts as a link.

Can you help me to learn more?

(Other definitions for string that I've seen before include "groups of horses" , "Group of horses" , "Collective noun for racehorses trained together" , "light cord" , "It's difficult to say how long a piece of it is" .)

I've seen this clue in the Evening Standard.
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