Line left in Scrabble regularly (5)
I believe the answer is:
cable
'line' is the definition.
(synonyms)
'left in scrabble regularly' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'scrabble' are 'cabe'.
'l' inserted within 'cabe' is 'CABLE'.
(Other definitions for cable that I've seen before include "Wire, flex" , "Telegram; thick rope" , "Telegraph" , "Wire; message" , "Communications wire" .)