One of the musketeers has to reform (5)
I believe the answer is:
'one of the musketeers' is the definition.
(I've seen this before)
'has to reform' is the wordplay.
'reform' indicates an anagram (letters in a new form).
'hasto' with letters rearranged gives 'ATHOS'.
(Other definitions for athos that I've seen before include "Brave soldier" , "Literary musketeer" , "Dumas musketeer" , "Greek monastic mountain" , "Greek mount; one of the Three Musketeers" .)