This falcon is among the sleekest, relatively speaking (7)
I believe the answer is:
kestrel
'this falcon' is the definition.
(kestrel is a kind of falcon)
'among the sleekest relatively speaking' is the wordplay.
'among' indicates a hidden word.
'KESTREL' is hidden in the letters of 'sleekest relatively speaking'.
'is' acts as a link.
(Other definitions for kestrel that I've seen before include "Bird of prey" , "Predator" , "High flier" , "Les, trek around the bird" , "It takes mice" .)