Trader going bust has to slow down (6)
I believe the answer is:
retard
'to slow down' is the definition.
(I know that retard is a more specific form of the action decelerate)
'trader going bust' is the wordplay.
'going bust' indicates anagramming the letters.
'trader' is an anagram of 'RETARD'.
'has' is the link.
(Other definitions for retard that I've seen before include "Check" , "Hold back, slow progress" , "Hold back (development)" , "Detain" , "Impede like trader" .)