Quiet at back of tunnel (5)
I believe the answer is:
shaft
'tunnel' is the definition.
(thesaurus)
'quiet at back' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'at' is a charade indicator (letters next to each other).
'back' becomes 'aft' (associated in meaning).
'sh'+'aft'='SHAFT'
'of' acts as a link.
(Other definitions for shaft that I've seen before include "Beam - well" , "Vertical excavation" , "Ray (of light), access to mine" , "Ray (of sunlight)" , "An arrow or a mine passage" .)