Birds slow in movement (4)
I believe the answer is:
owls
'birds' is the definition.
(owl is a kind of bird)
'slow in movement' is the wordplay.
'in movement' indicates an anagram.
'slow' is an anagram of 'OWLS'.
(Other definitions for owls that I've seen before include "Night birds" , "Nocturnal bird" , "Nocturnal hooters?" , "Avian symbols of wisdom" , "Types incredibly wise" .)