Bug, furious, keeps slowing down (8)
I believe the answer is:
irritate
'bug' is the definition.
(I've seen this in another clue)
'furious keeps slowing down' is the wordplay.
'furious' becomes 'irate' (I've seen this before).
'keeps' is an insertion indicator (inserted letters are kept inside).
'slowing down' becomes 'rit' (abbreviation for musical term ritardando).
'irate' placed around '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" .)