An accident messed up his map (6)
I believe the answer is:
mishap
'an accident' is the definition.
(mishap is a kind of accident)
'messed up his map' is the wordplay.
'messed up' indicates anagramming the letters.
'his'+'map'='hismap'
'hismap' anagrammed gives 'MISHAP'.
(Other definitions for mishap that I've seen before include "Contretemps" , "An accident, piece of bad luck" , "Accident, something going wrong" , "Blow" , "An accident or unlucky event" .)