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'.
'at' means one lot of letters go next to another.
'back' becomes 'aft' ('aft' is associated in meaning with 'back'**).
'sh'+'aft'='SHAFT'
'of' is the 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" .)