Party lines for plaything (4)
I believe the answer is:
doll
'plaything' is the definition.
(doll is a kind of plaything)
'party lines' is the wordplay.
'party' becomes 'do' (do can informally mean a party).
'lines' becomes 'll' (two abbreviations for line).
'do'+'ll'='DOLL'
'for' acts as a link.
(Other definitions for doll that I've seen before include "Play figure" , "Toy in human form" , "Small replica of a person" , "Girl's toy" , "Toy baby" .)