Act in a servile manner? Swimmer might do this (5)

I believe the answer is:

crawl

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'act' is the definition.
(crawl is a kind of act)

I can't explain the rest of the clue.

(Other definitions for crawl that I've seen before include "make slow progress" , "Go on all fours" , "fawn" , "Make no haste" , "Be obsequious; swimming stroke" .)

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