Legal action about soldiers in line (6)
I believe the answer is:
crease
'line' is the definition.
('crease' can be a synonym of 'line')
'legal action about soldiers' is the wordplay.
'legal action' becomes 'case' ('case' can be a synonym of 'legal action'**).
'about' indicates putting letters inside.
'soldiers' becomes 're' (Royal Engineers).
'case' enclosing 're' is 'CREASE'.
'in' acts as a link.
(Other definitions for crease that I've seen before include "Fold - wrinkle" , "Folded line" , "Where the batsman stands" , "feature of smart trousers" , "Line made by folding" .)