Knitting stitch (5)
'knitting stitch' is the definition.
(purl is a kind of knitting stitch)
This is all the clue.
(Other definitions for purl that I've seen before include "form loop" , "Twisted gold wire" , "Knitting stitch type" .)
(Other definitions for cable that I've seen before include "Liberal Democrat" , "Strong line" , "A message transmitted" , "Telegram; thick rope" , "Communications wire" .)