Union divides many boors (5)
I believe the answer is:
louts
'boors' is the definition.
(I've seen this in another clue)
'union divides many' is the wordplay.
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'divides' is an insertion indicator (inserted letters divide the word).
'many' becomes 'lots' (lots of something).
'u' going within 'lots' is 'LOUTS'.
(Other definitions for louts that I've seen before include "Dolts" , "Yobs" , "Coarse types" , "Uncouth aggressive men" .)