That's quite a lot for just a horse (4,6)

I believe the answer is:

fair amount

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'that's quite a lot' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'just a horse' is the wordplay.
'just' becomes 'fair' (just can mean fair in the sense of showing justice).
'horse' becomes 'mount' (mount is a kind of horse).
'fair'+'a'+'mount'='FAIR AMOUNT'

'for' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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