Know lettings house a fly-by-night (5)
I believe the answer is:
owlet
'a fly-by-night' is the definition.
(owls fly at night)
'know lettings house' is the wordplay.
'house' indicates a hidden word (I've seen 'houses' mean this (part of the clue houses the answer)).
'OWLET' can be found hidden inside 'know lettings'.
(Other definitions for owlet that I've seen before include "Young nocturnal bird of prey" , "Little nocturnal bird" , "Young nocturnal hunting bird" , "Offspring of the wise one?" , "Small hooter" .)