Rash on leg had developed (8)
I believe the answer is:
headlong
'rash' is the definition.
(thesaurus)
'on leg had developed' is the wordplay.
'developed' indicates anagramming the letters.
'on'+'leg'+'had'='onleghad'
'onleghad' with letters rearranged gives 'HEADLONG'.
(Other definitions for headlong that I've seen before include "not considered" , "In hasty or plunging manner" , "Hasty and rash, falling or plunging" , "Precipitate(ly)" , "Precipitate (flight)" .)