Crossword Genius

What the mind does when a horse appears in trunks? (7)

Ross

I believe the answer is:

boggles

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'what the mind does' is the definition.
The definition suggests a comparative adjective but the answer is not.

'a horse appears in trunks?' is the wordplay.
'a horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'appears in' means one lot of letters goes inside another.
'trunks?' becomes 'boles' (I've seen this in another clue).
'gg' put within 'boles' is 'BOGGLES'.

'when' acts as a link.

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I've seen this clue in the Evening Standard.
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