Her allies reunited to cause trouble (5,4)
I believe the answer is:
raise hell
'cause trouble' is the definition.
(to raise hell is to cause trouble or commotion)
'her allies reunited' is the wordplay.
'reunited' indicates an anagram (I've seen 'reuniting' mean this (letters are put together again in a new way)).
'her'+'allies'='herallies'
'herallies' anagrammed gives 'RAISE HELL'.
'to' acts as a link.
(Other definitions for raise hell that I've seen before include "Complain in the strongest possible terms" , "create discord" , "complain vociferously" , "cause trouble" , "create commotion" .)