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' says to put letters next to each other.
'party' becomes 'do' (do can informally mean a party).
'line' becomes 'l' (used when specifying particular lines from a poem).
'l' put 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" .)