Keep control, engaging volunteers once (6)
I believe the answer is:
retain
'keep' is the definition.
(I know that retain is a kind of keep)
'control engaging volunteers once' is the wordplay.
'control' becomes 'rein' (rein is a kind of control).
'engaging' means one lot of letters goes inside another (inserted letters are engaged or involved in others).
'volunteers once' becomes 'ta' (abbreviation for Territorial Army. I am not sure about the 'once' bit.).
'rein' enclosing 'ta' is 'RETAIN'.
Can you help me to learn more?
(Other definitions for retain that I've seen before include "Keep in place" , "Continue to hold on to" , "Save" , "'Hold, keep back (6)'" , "Hire - keep" .)