Keep Territorial Army in check (6)
I believe the answer is:
retain
'keep' is the definition.
(I know that keep can be written as retain)
'territorial army in check' is the wordplay.
'territorial army' becomes 'ta'.
'in' is an insertion indicator.
'check' becomes 'rein' (both can mean to control or limit).
'ta' put within 'rein' is 'RETAIN'.
(Other definitions for retain that I've seen before include "Hold in position" , "Keep possession of" , "'Hold, keep back (6)'" , "Keep, hold on to something" , "Keep in place" .)