Exceptional clue ... (6)
I believe the answer is:
signal
I believe this is a double definition.
'exceptional' is the first definition.
(thesaurus)
'clue' is the second definition.
(thesaurus)
(Other definitions for signal that I've seen before include "Gesture giving information or command" , "Nonverbal action or gesture that encodes a message" , "Message in the form of a gesture" , "Important; cue" , "Gesture or object giving information" .)