Restrain soldiers at home (4)
I believe the answer is:
rein
'restrain' is the definition.
(reining is a kind of restraining)
'soldiers at home' is the wordplay.
'soldiers' becomes 're' (Royal Engineers**).
'at home' becomes 'in' ('in' can be similar in meaning to 'at home').
're'+'in'='REIN'
(Other definitions for rein that I've seen before include "Harness item" , "Hold back" , "Strap to control horse" , "Animal controller" , "Free ... is carte blanche" .)