Fancy nuts in fast (4,5)

I believe the answer is:

like crazy

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'fast' is the definition.
Both the answer and definition are adverbs. Maybe they are linked in a way I don't understand?

'fancy nuts' is the wordplay.
'fancy' becomes 'like' (both can mean to find someone attractive).
'nuts' becomes 'crazy' (I've seen this in another clue).
'like'+'crazy'='LIKE CRAZY'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for like crazy that I've seen before include "Furiously" , "Very much" , "great enthusiasm" .)

I've seen this clue in The Independent.

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