Crossword Genius

In which a female, at middle age, can see herself (5)

Ross

I believe the answer is:

glass

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

'in which a female at middle age can see herself' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Maybe you can see an association between them that I can't see?

'in which a female' is the wordplay.
'in which' becomes 'g' (I can't explain this - if you can you should believe this answer much more).
'a female' becomes 'lass' (lass is a kind of female).
'g'+'lass'='GLASS'

Can you help me to learn more?

(Other definitions for glass that I've seen before include "that one can see through" , "Drink container" , "Balloon, say" , "(Material used in) mirror" , "Eg, tumbler" .)

I've seen this clue in the Evening Standard.
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