Arson involving navigation aid (5)
I believe the answer is:
sonar
'navigation aid' is the definition.
(I've seen this in another clue)
'arson involving' is the wordplay.
'involving' indicates anagramming the letters.
'arson' anagrammed gives 'SONAR'.
(Other definitions for sonar that I've seen before include "Echo-sounding equipment" , "echo device" , "Echo-location system" , "Echo-sounding device" , "Detection system at sea" .)