Notice the man at the top, a leader (4)
I believe the answer is:
head
'a leader' is the definition.
(head is a kind of leader)
'notice the man at the top' is the wordplay.
'notice' becomes 'ad'.
'the man' becomes 'he'.
'at the top' means one lot of letters go next to another.
'ad' after 'he' is 'HEAD'.
(Other definitions for head that I've seen before include "Froth on beer" , "See 14" , "Chief; go first" , "Promontory; foam" , "Skull" .)