His map is spoiled by accident (6)
I believe the answer is:
mishap
'accident' is the definition.
(mishap is a kind of accident)
'his map is spoiled' is the wordplay.
'is spoiled' is an anagram indicator.
'his'+'map'='hismap'
'hismap' with letters rearranged gives 'MISHAP'.
'by' acts as a link.
(Other definitions for mishap that I've seen before include "Unfortunate occurrence" , "Stroke of bad luck" , "Accident, unlucky happening" , "Accident, something going wrong" , "Unlucky incident" .)