TV (6-3)
'tv' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for goggle-box that I've seen before include "Television set" , "TV" .)
(Other definitions for plasma that I've seen before include "Hot ionised gas" , "Liquid part of blood" , "Colourless fluid in which blood cells are suspended" , "A psalm (anag.)" , "part of donor's contribution?" .)