Slow-moving birds (4)
I believe the answer is:
owls
'birds' is the definition.
(owl is a kind of bird)
'slow-moving' is the wordplay.
'moving' indicates an anagram.
'slow' with letters rearranged gives 'OWLS'.
(Other definitions for owls that I've seen before include "Slow (anag) -- birds of prey" , "Wise old birds - quite slow" , "Members of Parliament" , "Night birds" , "Types incredibly wise" .)