One hears man-at-arms in darkness (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a period of darkness)
'one hears man-at-arms' is the wordplay.
'one hears' indicates a 'sounds like' (homophone) clue (I've seen 'hearing' mean this).
'man-at-arms' becomes 'knight'.
'knight' is a homophone of 'NIGHT'.
'in' acts as a link.
(Other definitions for night that I've seen before include "Day's partner" , "Tennyson's "black bat"" , "Ignorance" , "Evening" , "See 1 Across" .)