Terribly slow birds (4)
I believe the answer is:
owls
'birds' is the definition.
(owl is a kind of bird)
'terribly slow' is the wordplay.
'terribly' indicates anagramming the letters.
'slow' with letters rearranged gives 'OWLS'.
(Other definitions for owls that I've seen before include "Nocturnal birds" , "Strigiform creatures" , "Slow (anag) -- birds of prey" , "Night birds" , "Nocturnal hooters?" .)