Keep the Territorial Army in control (6)
I believe the answer is:
retain
'keep' is the definition.
(I know that keep can be written as retain)
'territorial army in control' is the wordplay.
'territorial army' becomes 'ta'.
'in' indicates putting letters inside.
'control' becomes 'rein' (reining is a kind of controlling).
'ta' placed within 'rein' is 'RETAIN'.
'the' is the link.
(Other definitions for retain that I've seen before include "Keep possession of" , "Continue to hold on to" , "Keep; remember" , "Hold onto" , "Keep, hold on to something" .)