Sounds as though it's bedtime, sir (6)
I believe the answer is:
knight
'sir' is the definition.
(knights have the title 'sir')
'sounds as though it's bedtime' is the wordplay.
I cannot really see how this works, but
'bedtime' could be 'night' and 'night' is present in the answer.
A single letter 'k' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for knight that I've seen before include "... for example, Tippett" , "Gawain?" , "eg Elton John" , "Holder of non-hereditary title in UK" , "Chesspiece - person dubbed" .)