Liberal following party line is plaything (4)
I believe the answer is:
doll
'is plaything' is the definition.
(I've seen this in another clue)
'liberal following party line' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'following' means one lot of letters go next to another.
'party' becomes 'do' (do can informally mean a party).
'line' becomes 'l' (used when specifying particular lines from a poem).
'l' after 'do' is 'dol'.
'dol'+'l'='DOLL'
(Other definitions for doll that I've seen before include "Child's plaything" , "Play figure" , "A girl's plaything" , "Child's toy figure" , "Toy baby" .)