Recipe in back reviewed and not cooked so well (5)

I believe the answer is:

rarer

'not cooked so well' is the definition.

I cannot understand how the remainder of the clue works.

(Other definitions for rarer that I've seen before include "Comparatively sparse" , "More unusual" , "Less frequent" , "Less often seen" , "Thinner" .)

I've seen this clue in The Guardian.

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