Conservative's leader Liberal prematurely? Obviously! (7)
I believe the answer is:
clearly
'obviously' is the definition.
(thesaurus)
'conservative's leader liberal prematurely?' is the wordplay.
'leader' suggests taking the first letters.
'liberal' becomes 'l' (abbreviation).
'prematurely?' becomes 'early' (I've seen this before).
The initial letter of 'conservatives' is 'c'.
'c'+'l'+'early'='CLEARLY'
(Other definitions for clearly that I've seen before include "Without doubt or question" , "Obviously; with clarity" .)