Man-at-arms announced time to retire (5)
I believe the answer is:
night
'time to retire' is the definition.
(I've seen this in another clue)
'man-at-arms announced' is the wordplay.
'man-at-arms' becomes 'knight'.
'announced' indicates a 'sounds like' (homophone) clue.
'knight' sounds like 'NIGHT'.
(Other definitions for night that I've seen before include "Darkness; see you in the morning" , "Tennyson's "black bat"" , "Time without 10" , "follow-up to 16 [TWILIGHT]" , "See 9" .)