Film posh Young Conservative dancing (6)
I believe the answer is:
psycho
'film' is the definition.
(I've seen this before)
'posh young conservative dancing' is the wordplay.
'young' becomes 'y'.
'conservative' becomes 'C' (abbreviation commonly used in UK politics).
'dancing' is an anagram indicator.
'posh'+'y'+'c'='poshyc'
'poshyc' is an anagram of 'PSYCHO'.
(Other definitions for psycho that I've seen before include "Nineteen sixty Hitchcock film" , "1960 horror film" , "1960 thriller starring a deranged Anthony Perkins" , "Chopsy (anag.)" , "unstable person" .)