The cue is very important (6)
I believe the answer is:
signal
I believe this clue is a double definition.
'the cue' is the first definition.
(I've seen this in another clue)
'very important' is the second definition.
(I've seen this before)
'is' is the link.
(Other definitions for signal that I've seen before include "Gesture with meaning" , "Gesture to convey information" , "intimation" , "Important; cue" , "Gesture; notable" .)