Stretcher bearer going the wrong way? What an idiot! (3)
I believe the answer is:
doh
'what an idiot' is the definition.
('d'oh' is said after doing something stupid)
'stretcher bearer going the wrong way?' is the wordplay.
'stretcher bearer' becomes 'hod' (hods carry bricks - stretcher is a type of brick).
'going the wrong way?' says the letters should be written in reverse.
'hod' in reverse letter order is 'DOH'.
(Other definitions for doh that I've seen before include "for 7 [HOMER] of course!" , "Homeric grunt of annoyance" , "whoops!" , "The note C" , "Silly me!" .)