Enlist perhaps but must be quiet (6)
I believe the answer is:
silent
'must be quiet' is the definition.
'silent' can be an answer for 'quiet' (I've seen this before). I'm not certain of the 'must be' bit.
'enlist perhaps' is the wordplay.
'perhaps' indicates an anagram.
'enlist' is an anagram of 'SILENT'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for silent that I've seen before include "Refusing to communicate" , "Enlist (anag) - mum" , "Noiseless" , "Free from noise" , "Quiet -- 13 (anag)" .)