Engineers and soldiers following soldier's rules (7)
I believe the answer is:
regimen
'rules' is the definition.
(a regimen is a system or set of rules)
'engineers and soldiers following soldier's' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'and' says to put letters next to each other.
'soldiers' becomes 'men' (I've seen this before).
'following' says to put letters next to each other.
'soldier' becomes 'gi' (GI can mean a US soldier).
'men' put after 'gi' is 'gimen'.
're'+'gimen' is 'REGIMEN'.
(Other definitions for regimen that I've seen before include "Method of working" , "Therapeutic programme (of diet and exercise)" , "Merge in for prescribed way of life" , "administration" , "Course of diet, therapy" .)