Trader upset by delay (6)
I believe the answer is:
retard
'delay' is the definition.
(I've seen this before)
'trader upset' is the wordplay.
'upset' is an anagram indicator.
'trader' anagrammed gives 'RETARD'.
'by' is the link.
(Other definitions for retard that I've seen before include "Slow down for trader" , "Hold back (development)" , "Check" , "Darted about to hold back or delay" , "Slow the progress of, delay" .)