Wayward
(Other definitions for headstrong that I've seen before include "Obstinately rash" , "pig-headed" , "strong-willed" , "Stubborn" , "Obstinate" .)
'wayward' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for errant that I've seen before include "Sinful" , "Like many a knight" , "Behaving badly" , "like Don Quixote?" , "Wayward - ranter (anag)" .)