A strange thing, but it's time for bed (5)
I believe the answer is:
night
'it's time for bed' is the definition.
(when people sleep)
'a strange thing' is the wordplay.
'a strange' indicates anagramming the letters.
'thing' is an anagram of 'NIGHT'.
'but' acts as a link.
(Other definitions for night that I've seen before include "'The 2 8 of the dog in the _____'" , "Time for turning" , "Time of darkness" , "See 1 Across" , "The dark time" .)