Wilt like school silver (3)

I believe the answer is:

sag

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'wilt' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

(Other definitions for sag that I've seen before include "Lose energy" , "Cave in" , "Give in the middle" , "Sink (under load)" , "Sink, hang down" .)

I've seen this clue in The Mirror.

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