Hefty type of cable split across diameter (4,4)
I believe the answer is:
bold face
'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).
'of'+'cable'='ofcable'
'ofcable' is an anagram of 'bolface'.
'bolface' enclosing 'd' is 'BOLD FACE'.
(Other definitions for bold face that I've seen before include "impudence?" , "Printing type with thick, heavy lines" , "Weight of type" , "Thick characters" , "one defiantly puts on" .)