Start old-fashioned joke (6,3)
I believe the answer is:
square one
'start' is the definition.
(thesaurus)
'old-fashioned joke' is the wordplay.
'old fashioned' becomes 'square'.
'joke' becomes 'one' (I've seen this before).
'square'+'one'='SQUARE ONE'
(Other definitions for square one that I've seen before include "back here after failure" , "Back to . . . . . . . . . - at starting point with no progress made" .)