Leader has ache, at full speed (8)
I believe the answer is:
headlong
'at full speed' is the definition.
(I've seen this before)
'leader has ache' is the wordplay.
'leader' becomes 'head' (head is a kind of leader**).
'has' is a charade indicator (letters next to each other).
'ache' becomes 'long' ('long' can be a synonym of 'ache').
'head'+'long'='HEADLONG'
(Other definitions for headlong that I've seen before include "At breakneck speed" , "great haste" , "Quickly" , "Precipitate (flight)" , "In a hasty or rash way - falling thus?" .)