Quiet, back in tunnel (5)
I believe the answer is:
shaft
'tunnel' is the definition.
(I know that shaft is a type of tunnel)
'quiet back' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'back' becomes 'aft' (associated in meaning).
'sh'+'aft'='SHAFT'
'in' acts as a link.
(Other definitions for shaft that I've seen before include "Mine access; handle" , "Arrow part" , "Stem, handle; (light) ray" , "Ray (of light), access to mine" , "Vertical excavation" .)