Precious little possibility of putting on weight? (3,6)

I believe the answer is:

fat chance

'precious little possibility' is the definition.

I cannot understand how the rest of the clue works.

(Other definitions for fat chance that I've seen before include "It will never happen!" , "No likelihood at all!" , "that won't happen" , "Little prospect" , "Slim hope, paradoxically!" .)

I've seen this clue in The Times.

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