Beam that's essentially normal in telecommunications (6)
I believe the answer is:
lintel
'beam' is the definition.
(lintel is a kind of beam)
'essentially normal in telecommunications' is the wordplay.
'essentially' indicates the answer is hidden within the clue (essence can mean the inside of something).
'LINTEL' is hidden in the letters of 'normal in telecommunications'.
'that's' acts as a link.
(Other definitions for lintel that I've seen before include "Horizontal timber support" , "Supporting member" , "It's found above the door" , "Supporting beam over a doorway" , "Part of door or window frame" .)