Arson involved a navigation aid (5)
I believe the answer is:
sonar
'a navigation aid' is the definition.
(I've seen this in another clue)
'arson involved' is the wordplay.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'arson' anagrammed gives 'SONAR'.
(Other definitions for sonar that I've seen before include "Underwater echo sounder" , "location device" , "Device for detecting objects used in arson" , "Method of echolocation" , "Locating device" .)