Keep volunteers in check (6)
I believe the answer is:
retain
'keep' is the definition.
(retaining is a kind of keeping)
'volunteers in check' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'in' indicates putting letters inside.
'check' becomes 'rein' (both can mean to control or limit).
'ta' going into 'rein' is 'RETAIN'.
(Other definitions for retain that I've seen before include "Keep; remember" , "Keep in Tirane" , "'Hold, keep back (6)'" , "Hold in position" , "Keep in place" .)