Concurred with mongrel (3)
I believe the answer is:
cur
'mongrel' is the definition.
(synonyms)
'concurred with' is the wordplay.
'with' indicates a hidden word.
'CUR' can be found hidden inside 'concurred'.
(Other definitions for cur that I've seen before include "Worthless mongrel" , "Worthless or bad-tempered dog" , "Mongrel - coward" , "Mangy dog" , "Aggressive mongrel" .)