Nosy substitute leader of district council (5)
I believe the answer is:
synod
'council' is the definition.
(I know that synod is a type of council)
'nosy substitute leader of district' is the wordplay.
'substitute' is an anagram indicator.
'leader of' indicates taking the first letters.
The first letter of 'district' is 'd'.
'nosy' with letters rearranged gives 'syno'.
'syno'+'d'='SYNOD'
(Other definitions for synod that I've seen before include "Church council" , "Ecumenical council" , "Clerical assembly" , "Council of clergy to discuss policy" , "group of clergymen" .)