Marshal aligns indicator (6)
I believe the answer is:
signal
'indicator' is the definition.
(I know that indicator can be written as signal)
'marshal aligns' is the wordplay.
'marshal' indicates anagramming the letters (I've seen 'marshals' mean this (marshal can mean to gather)).
'aligns' with letters rearranged gives 'SIGNAL'.
(Other definitions for signal that I've seen before include "Non-verbal action with an encoded message" , "Communication that encodes a message" , "Outstanding - information sent over a distance" , "Indication or warning" , "Sign that conveys a message" .)