Slow-moving flyers (4)
I believe the answer is:
owls
'flyers' is the definition.
(owls are things that fly)
'slow-moving' is the wordplay.
'moving' indicates an anagram.
'slow' is an anagram of 'OWLS'.
(Other definitions for owls that I've seen before include "Slow kind of birds" , "Wise old birds - quite slow" , "Types incredibly wise" , "Slow sort of birds" , "Strigiform creatures" .)