Hefty type of cable split across diameter (4,4)
I believe the answer is:
Here is my best explanation:
'hefty type' is the definition.
'of cable split across diameter' is the wordplay.
'split' indicates anagramming the letters.
'across' indicates putting letters inside.
'diameter' becomes 'd' (mathematics).
'ofcable' is an anagram of 'bolface'.
'bolface' enclosing 'd' is 'BOLD FACE'.
(Other definitions for bold face that I've seen before include "prominently visible", "Printing type with thick, heavy lines", "one defiantly puts on", "impudence?", "A type", "weight", "Weight of type", "Thick characters" and "Type".)