Jumper showing signs of exertion? (7)

I believe the answer is:

sweater

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But I can't explain it at all! Could you help me understand?

(Other definitions for sweater that I've seen before include "mugger once" , "One visibly too hot" , "Jumper (7)" , "Long-sleeved pullover" , "Heavy jersey" .)

I've seen this clue in The Mirror.

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