Soldiers in favour of mass change for the better (6)
I believe the answer is:
reform
'change for the better' is the definition.
('reform' can be a synonym of 'change for the better')
'soldiers in favour of mass' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'in favour of' becomes 'for' (I've seen this before).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
're'+'for'+'m'='REFORM'
(Other definitions for reform that I've seen before include "Show improvement" , "Change one's ways (for the better?)" , "Mend one's ways" , "Go straight" , "Abandon evil practices" .)