Power gap filled by party leader (9)
I believe the answer is:
president
'leader' is the definition.
(type of leader)
'power gap filled by party' is the wordplay.
'power gap' becomes 'prent' (I can't justify this - if you can you should believe this answer much more).
'filled by' indicates putting letters inside.
'party' becomes 'side' (eg a side or party in a contract).
'prent' going around 'side' is 'PRESIDENT'.
Can you help me to learn more?
(Other definitions for president that I've seen before include "Our head of state has a strange red instep" , "Head of college" , "Cleveland?" , "Country's leader" , "such as Bush" .)