Signal functioning beyond stream (6)
I believe the answer is:
beckon
'signal' is the definition.
(I've seen this before)
'functioning beyond stream' is the wordplay.
'functioning' becomes 'on' (I've seen this in other clues).
'beyond' means one lot of letters go next to another.
'stream' becomes 'beck' (northern English dialect term for 'stream').
'on' after 'beck' is 'BECKON'.
(Other definitions for beckon that I've seen before include "Gesture closer" , "Summon by gesturing" , "Summon by sign" , "Make signs to summon" , "Gesture to approach" .)