More lean than is immediately necessary? (5)

I believe the answer is:

spare

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'more' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for spare that I've seen before include "This tyre around the middle is fat" , "Surplus to requirements" , "Extra; lean" , "Excess to requirements" , "A feature of trousers" .)

I've seen this clue in the Evening Standard.

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