Reckless leader to be dying (8)
I believe the answer is:
headlong
'reckless' is the definition.
(thesaurus)
'leader to be dying' is the wordplay.
'leader' becomes 'head' (head is a kind of leader).
'to be dying' becomes 'long' (to be dying for something is to long for it).
'head'+'long'='HEADLONG'
(Other definitions for headlong that I've seen before include "In a hasty or rash way - falling thus?" , "Rashly" , "Precipitous, rash" , "Precipitate(ly)" , "In hasty or plunging manner" .)