Shyness shown by potential substitute (7)

I believe the answer is:

reserve

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This might be a double definition.

'shyness' is the first definition.

'potential substitute' is the second definition.
'reserve' can be an answer for 'substitute' (synonyms). I am not sure about the 'potential' bit.

'shown by' acts as a link.

Can you help me to learn more?

(Other definitions for reserve that I've seen before include "Bank" , "Keep back - book" , "Conservation area" , "Wine kept" , "Modesty- store" .)

I've seen this clue in The Irish Independent.

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