Arson solved by navigation aid (5)
I believe the answer is:
sonar
'navigation aid' is the definition.
(I've seen this in another clue)
'arson solved' is the wordplay.
'solved' is an anagram indicator (the anagram must be solved).
'arson' with letters rearranged gives 'SONAR'.
'by' is the link.
(Other definitions for sonar that I've seen before include "Underwater navigation device" , "Underwater detection system" , "Underwater location equipment" , "Echo-location system" , "Echo sounder - arson (anag)" .)