Box enthusiast likes a reddish-brown mount (8)

I believe the answer is:

chestnut

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

'likes a reddish-brown mount' is the definition.
I can't tell whether this defines the answer.

'box enthusiast' is the wordplay.
'box' becomes 'chest' (chest is a kind of box).
'enthusiast' becomes 'nut' (nut can mean an enthusiast or fan).
'chest'+'nut'='CHESTNUT'

Can you help me to learn more?

(Other definitions for chestnut that I've seen before include "Brown horse" , "''Underneath the spreading . . . . . . . . tree''" , "Laughably predictable" , "Wooden" , "Golden to reddish brown colour" .)

I've seen this clue in the Sydney Morning Herald.

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