Keep the Terriers in check! (6)
I believe the answer is:
retain
'keep' is the definition.
(retaining is a kind of keeping)
'terriers in check' is the wordplay.
'terriers' becomes 'ta' (nickname for Territorial Army members).
'in' is an insertion indicator.
'check' becomes 'rein' (both can mean to control or limit).
'ta' placed inside 'rein' is 'RETAIN'.
'the' acts as a link.
(Other definitions for retain that I've seen before include "Keep possession of" , "Hold on to or employ counsel" , "Keep, hold on to something" , "Hold onto" , "Keep in place" .)