Boys at the back imitate cattle, which is a bit dim-witted (4)

I believe the answer is:

slow

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'dim-witted' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for slow that I've seen before include "Signal to follow" , "Leisurely" , "Not getting a move on" , "Crawling" , "Sluggish" .)

I've seen this clue in The Irish Times.

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