Soldier keeps to rules I lay down (9)
I believe the answer is:
regulator
'rules i lay down' is the definition.
(someone who lays down rules or regulations)
'soldier keeps to' is the wordplay.
'soldier' becomes 'regular' (member of the regular army).
'keeps' indicates putting letters inside.
'regular' enclosing 'to' is 'REGULATOR'.
(Other definitions for regulator that I've seen before include "Of body" , "source of rules?" , "part of the watch" , "Ofsted perhaps" , "Such as OFGAS, OFTEL etc" .)