Keep TA in control (6)
I believe the answer is:
retain
'keep' is the definition.
(retaining is a kind of keeping)
'ta in control' is the wordplay.
'in' indicates putting letters inside.
'control' becomes 'rein' (rein is a kind of control).
'ta' put into 'rein' is 'RETAIN'.
(Other definitions for retain that I've seen before include "Hold onto" , "Keep in place" , "'Hold, keep back (6)'" , "Not relinquish" , "Hold in position" .)