Two leaders meeting hastily (9)
I believe the answer is:
headfirst
'hastily' is the definition.
(thesaurus)
'two leaders meeting' is the wordplay.
'two leaders' means two replacements for 'leader'.
'leader' becomes 'head' (head is a kind of leader).
'leader' becomes 'first'.
'meeting' is a charade indicator (letters next to each other).
'head'+'first'='HEADFIRST'
(Other definitions for headfirst that I've seen before include "Without thinking" , "Impetuously" , "rashly" , "If trashed, dive as divers usually do" .)