Cut back troops initially on border (4)
I believe the answer is:
trim
'cut back' is the definition.
('trim' can be a synonym of 'cut back')
'troops initially on border' is the wordplay.
'initially' suggests taking the first letters.
'on' says to put letters next to each other.
'border' becomes 'rim' (I've seen this before**).
The first letter of 'troops' is 't'.
't'+'rim'='TRIM'
(Other definitions for trim that I've seen before include "Cut, prune; decorate" , "Make smooth by cutting away irregular parts" , "Neat and ticy in Meath" , "Dress - prune" , "fittings" .)