Delay upsetting trader (6)
I believe the answer is:
retard
'delay' is the definition.
(I know that delay can be written as retard)
'upsetting trader' is the wordplay.
'upsetting' indicates anagramming the letters.
'trader' with letters rearranged gives 'RETARD'.
(Other definitions for retard that I've seen before include "Detain" , "Hold back, slow progress" , "Hold back (development)" , "Hold back -- trader (anag)" , "Impede like trader" .)