He overrules his brothers (5)
I believe the answer is:
abbot
'his brothers' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he overrules' is the wordplay.
'he' becomes 'Ab'.
'overrules' becomes 'bot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ab'+'bot'='ABBOT'
Can you help me to learn more?
(Other definitions for abbot that I've seen before include "friar" , "Head of group of monks" , "chap who'll be over order?" , "Supervisor of brothers" , "Convent leader, male" .)