Rubbish Conservative on knees by ambush (8)
I believe the answer is:
claptrap
'rubbish' is the definition.
(thesaurus)
'conservative on knees by ambush' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'on' means one lot of letters go next to another.
'knees' becomes 'lap' (I've seen this before).
'by' says to put letters next to each other.
'ambush' becomes 'trap' ('trap' can be a synonym of 'ambush').
'c'+'lap'+'trap'='CLAPTRAP'
(Other definitions for claptrap that I've seen before include "moonshine" , "Stuff and nonsense" , "High-sounding nonsense" , "Foolish or pretentious talk" , "Meaningless, insincere or pompous talk" .)