Ardent Conservative overlooked, moderate leader brought in (9)
I believe the answer is:
committed
'ardent' is the definition.
(thesaurus)
'conservative overlooked moderate leader brought in' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'overlooked' becomes 'omitted' ('omit' can be a synonym of 'overlook'**).
'leader' suggests taking the first letters.
'brought in' is an insertion indicator.
The initial letter of 'moderate' is 'm'.
'omitted' enclosing 'm' is 'ommitted'.
'c'+'ommitted'='COMMITTED'
(Other definitions for committed that I've seen before include "Bound or obligated" , "Dedicated" , "Carried out" , "Performed an act" , "put away" .)