Signal before entrance to every junction (4)
I believe the answer is:
node
'junction' is the definition.
(I've seen this before)
'signal before entrance to every' is the wordplay.
'signal' becomes 'nod' (I have seen 'Signal of assent' mean 'nod' so perhaps 'signal' could also mean 'nod').
'before' is a charade indicator (letters next to each other).
'entrance to' indicates taking the first letters.
The initial letter of 'every' is 'e'.
'nod'+'e'='NODE'
(Other definitions for node that I've seen before include "Done to a small lump" , "Network junction" , "growth" , "part of network" , "Point of branching in a network" .)