Some leavers reform (7)
I believe the answer is:
several
'some' is the definition.
(similar in meaning)
'leavers reform' is the wordplay.
'reform' is an anagram indicator (letters in a new form).
'leavers' is an anagram of 'SEVERAL'.
(Other definitions for several that I've seen before include "Reveals (anag.)" , "A few; separate" , "More than one or two" , "three or four" , "quite a lot" .)