Navigation aid solved arson (5)
I believe the answer is:
sonar
'navigation aid' is the definition.
(I've seen this in another clue)
'solved arson' is the wordplay.
'solved' is an anagram indicator (the anagram must be solved).
'arson' with letters rearranged gives 'SONAR'.
(Other definitions for sonar that I've seen before include "Underwater detection system" , "Underwater navigation device" , "Underwater detection device" , "location device" , "System for locating underwater objects" .)