Sullivan's partner given a new start, the nut (7)

I believe the answer is:

filbert

'the nut' is the definition.
(filbert is a kind of nut)

I can't explain the remainder of the clue.

(Other definitions for filbert that I've seen before include "Type of hazelnut" , "Hazel nut" , "Nut -- left rib (anag)" .)

I've seen this clue in The Times.

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