Slash, old, again playing (8)
I believe the answer is:
diagonal
'slash' is the definition.
(synonyms)
'old again playing' is the wordplay.
'playing' indicates an anagram.
'old'+'again'='oldagain'
'oldagain' is an anagram of 'DIAGONAL'.
(Other definitions for diagonal that I've seen before include "straight line from corner" , "Slanting line joining two opposite corners" , "Sloping line" , "Corner-to-corner line" , "Line connecting two nonadjacent corners of a rectangle" .)