Quiet, rearward part of a mine? (5)
I believe the answer is:
shaft
'part of a mine?' is the definition.
('shaft' is part of 'mine')
'quiet rearward' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'rearward' becomes 'aft' (I've seen this in another clue).
'sh'+'aft'='SHAFT'
(Other definitions for shaft that I've seen before include "Vertical mine access" , "Long rod or column of light" , "Ray; access hole to a mine" , "Beam - well" , "Long slender part or deep vertical opening" .)