Owls fly, but not fast (4)
I believe the answer is:
slow
'not fast' is the definition.
('slow' is the opposite to 'fast')
'owls fly' is the wordplay.
'fly' is an anagram indicator.
'owls' is an anagram of 'SLOW'.
'but' is the link.
(Other definitions for slow that I've seen before include "Lose velocity" , "(Clock) behind" , "Reluctant" , "Not getting a move on" , "Long-drawn-out; 7 down" .)