Rule restricting soldier's first leave (6)
I believe the answer is:
resign
'leave' is the definition.
(I've seen this before)
'rule restricting soldier's first' is the wordplay.
'rule' becomes 'reign' (reign is a kind of rule).
'restricting' is an insertion indicator.
'soldier's first' becomes 's' (1st letter of 'soldier').
'reign' going around 's' is 'RESIGN'.
(Other definitions for resign that I've seen before include "admit defeat" , "Give up one's post" , "surrender" , "Give up office" , "Leave office voluntarily" .)