Recruit is silent when disciplined (6)
I believe the answer is:
enlist
'recruit' is the definition.
(enlisting is a kind of recruiting)
'silent when disciplined' is the wordplay.
'when disciplined' indicates anagramming the letters (I've seen this in other clues).
'silent' is an anagram of 'ENLIST'.
'is' acts as a link.
(Other definitions for enlist that I've seen before include "Join the military" , "Join the armed services" , "Join the armed forces" , "Enrol in armed forces" , "Register" .)