Dark near bottom of shaft (5)
I believe the answer is:
night
'dark' is the definition.
(it's dark at nighttime)
'near bottom of shaft' is the wordplay.
'near' becomes 'nigh' (synonyms).
'bottom of' says to take the final letters.
The last letter of 'shaft' is 't'.
'nigh'+'t'='NIGHT'
(Other definitions for night that I've seen before include "Opposite of day" , "Thing (anag)" , "Time for bed" , "Saturday -- Fever (film)" , "Tennyson's "black bat"" .)