Bug, furious, keeps slowing down (8)
I believe the answer is:
irritate
'bug' is the definition.
(thesaurus)
'furious keeps slowing down' is the wordplay.
'furious' becomes 'irate' (I've seen this before).
'keeps' is an insertion indicator.
'slowing down' becomes 'rit' (I've seen this in other clues).
'irate' enclosing 'rit' is 'IRRITATE'.
(Other definitions for irritate that I've seen before include "Cause annoyance" , "Annoy or cause itching" , "Wind up" , "Rattier, I may be annoying" , "bug" .)