Non-verbal communication (6)
'non-verbal communication' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for signal that I've seen before include "Indication; a train control" , "important to train drivers > striking" , "Gesture with meaning" , "Indicate" , "Aligns (anag) -- gesture" .)
(Other definitions for body language that I've seen before include "Unspoken communication" , "suggestive poses?" , "Non-verbal communication by posture, gestures etc" , "Reading material" , "means of communication?" .)