Leader of union forcing rebellion (8)
I believe the answer is:
uprising
'rebellion' is the definition.
(I know that rebellion can be written as uprising)
'leader of union forcing' is the wordplay.
'leader of' indicates taking the first letters.
'forcing' becomes 'prising' (as in prising something open).
The first letter of 'union' is 'u'.
'u'+'prising'='UPRISING'
(Other definitions for uprising that I've seen before include "Rebellion, revolt" , "Insurrection, rebellion" , "Open rebellion" , "Organised opposition to authority" , "Mutiny" .)