Arson involved navigation aid (5)
I believe the answer is:
sonar
'navigation aid' 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 "Device for detecting objects used in arson" , "Underwater navigation device" , "Means of detecting" , "Echo-sounding equipment" , "Underwater detection device" .)