Arson involved a navigation device (5)
I believe the answer is:
sonar
'a navigation device' is the definition.
(I've seen this in another clue)
'arson involved' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'arson' is an anagram of 'SONAR'.
(Other definitions for sonar that I've seen before include "location device" , "Underwater detection system" , "Device for detecting objects under water with sound waves" , "Echo-location system" , "Echo-sounding device" .)