Old, not very bright, full of universal hatred (5)
I believe the answer is:
odium
'hatred' is the definition.
(I know that odium is a type of hatred)
'old not very bright full of universal' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'not very bright' becomes 'dim' (I've seen this before).
'full of' means one lot of letters goes inside another.
'universal' becomes 'U' (film certificate).
'dim' placed around 'u' is 'dium'.
'o'+'dium'='ODIUM'
(Other definitions for odium that I've seen before include "Widespread dislike or disgust" , "Offensiveness" , "Execration" , "Disfavour" , "that would cause disgust" .)