Leader seeing heyday in title outside (5,8)
I believe the answer is:
prime minister
'leader' is the definition.
(prime minister is a kind of leader)
'heyday in title outside' is the wordplay.
'heyday' becomes 'prime' (synonyms).
'title' becomes 'mister'.
'outside' means one lot of letters goes inside another.
'in' going inside 'mister' is 'minister'.
'prime'+'minister'='PRIME MINISTER'
'seeing' acts as a link.
(Other definitions for prime minister that I've seen before include "PM" , "May" , "North, for instance" , "Head of state" , "Law once in UK" .)