Weakling who hisses a lot? (5)

I believe the answer is:

sissy

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'weakling' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for sissy that I've seen before include "Effeminate person" , "Feeble and effeminate" , "wet" , "Effeminate and over-timorous male" , "Weakling" .)

I've seen this clue in the Evening Standard.

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