System for detecting characters involved in arson, repeatedly (5)
I believe the answer is:
sonar
'system for detecting' is the definition.
'in arson repeatedly' is the wordplay.
'arson repeatedly' means two substitutions for 'arson'.
'in' says the answer is hidden in the clue.
'SONAR' is hidden within 'arson arson'.
'characters involved' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for sonar that I've seen before include "Underwater echo sounder" , "Underwater detection device" , "Device for detecting objects under water with sound waves" , "location device" , "Undersea sensing system" .)