Fatheaded beast, but understood to be sly (3)

I believe the answer is:

fox

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'understood to be sly' is the definition.
I can't judge whether this definition defines the answer.

'fatheaded beast' is the wordplay.
'fatheaded' becomes 'f' (this might be a standard abbreviation of which I'm unaware).
'beast' becomes 'ox'.
'f'+'ox'='FOX'

'but' is the link.

Can you help me to learn more?

(Other definitions for fox that I've seen before include "Hunted animal" , "Sly type" , "Puzzle" , "earth-dweller" , "Vulpine creature" .)

I've seen this clue in the Evening Standard.

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