Dislike old, stupid union being included (5)
I believe the answer is:
odium
'dislike' is the definition.
(I've seen this before)
'old stupid union being included' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'stupid' becomes 'dim' (I've seen this before).
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'being included' indicates putting letters inside.
'dim' enclosing 'u' is 'dium'.
'o'+'dium'='ODIUM'
(Other definitions for odium that I've seen before include "Execration" , "Widespread hatred or abhorrence (5)" , "Widespread hatred or disgust felt towards person" , "shame!" , "reproach" .)