Outspoken opposition from Munich cardinal (4)
I believe the answer is:
nine
'cardinal' is the definition.
(a cardinal number)
'outspoken opposition from munich' is the wordplay.
'outspoken' indicates a 'sounds like' (homophone) clue.
'opposition from munich' becomes 'nein' ('no' in German).
'nein' sounds like 'NINE'.
(Other definitions for nine that I've seen before include "Square next up after four" , "Number of players in a baseball fielding team" , "Three squared" , "and 23: Flash in the pan" , "Short time for a wonder" .)