Contemptuous cry about Liberal claptrap (4)
I believe the answer is:
blah
'claptrap' is the definition.
('blah' can be a synonym of 'claptrap')
'contemptuous cry about liberal' is the wordplay.
'contemptuous cry' becomes 'bah' (as in 'bah, humbug'**).
'about' is an insertion indicator.
'liberal' becomes 'l' (abbreviation).
'bah' going around 'l' is 'BLAH'.
(Other definitions for blah that I've seen before include "Pretentious nonsense" , "Rhubarb" , "Pretentious waffle" , "Claptrap" , "Dull" .)