Men's faith is wavering in underground passage (9)
I believe the answer is:
mineshaft
'underground passage' is the definition.
'men's faith is wavering' is the wordplay.
'is wavering' is an anagram indicator.
'mens'+'faith'='mensfaith'
'mensfaith' is an anagram of 'MINESHAFT'.
'in' acts as a link.
(Other definitions for mineshaft that I've seen before include "one's got bored" , "not a feature of 4, 21 [OPENCAST MINING]" , "Vertical hole" .)