Party includes Liberal creep (5)
I believe the answer is:
sidle
'creep' is the definition.
(thesaurus)
'party includes liberal' is the wordplay.
'party' becomes 'side' (I've seen this before**).
'includes' means one lot of letters goes inside another.
'liberal' becomes 'l' (abbreviation).
'side' enclosing 'l' is 'SIDLE'.
(Other definitions for sidle that I've seen before include "Isled (anag.)" , "Walk sideways in furtive manner" , "Move in furtive or timid manner" , "sidewalk" , "Edge along laterally" .)