A fan who's tough to crack (3)

I believe the answer is:

nut

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'a fan' is the definition.
(both can mean an enthusiast)

I don't understand the remainder of the clue.

(Other definitions for nut that I've seen before include "Crazy person" , "Fruitcake" , "Mechanical fastener" , "Head; hard fruit" , "Vegetarian food" .)

I've seen this clue in the Evening Standard.

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