Unhappy as your solution won't fit like this! (4)

I believe the answer is:

down

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'unhappy' is the definition.
(down can mean sad)

I cannot understand how the rest of the clue works.

(Other definitions for down that I've seen before include "In a low mood; hill" , "Melancholy" , "Rolling hill; soft feathers" , "Not working (computer)" , "See 10" .)

I've seen this clue in The Guardian.

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