Girl disheartened for yonks (4)

I believe the answer is:

ages

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'yonks' is the definition.
(I've seen this before)

'girl disheartened' is the wordplay.
'girl' becomes 'Ag'.
'disheartened' becomes 'es' (I can't justify this - if you can you should give a lot more credence to this answer).
'ag'+'es'='AGES'

'for' acts as a link.

Can you help me to learn more?

(Other definitions for ages that I've seen before include "Sage (anag)" , "Matures" , "(What seems) a very long time" , "Donkey's years" , "Epochs" .)

I've seen this clue in The Sun.

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