Crossword Genius

One increasing guineas or shillings? (8)

Ross

I believe the answer is:

extender

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

'one increasing guineas or shillings?' is the definition.
The definition suggests a singular noun which matches the answer.

'one' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'er' (I've seen this in other clues) and 'er' is found within the answer.
The remaining letters 'extend' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for extender that I've seen before include "stretcher required?" , "one to reach out" .)

I've seen this clue in The Independent.
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