Bird of prey. Some of the quickest rely on surprise (7)
I believe the answer is:
kestrel
'bird of prey' is the definition.
(kestrel is a kind of bird of prey)
'some of the quickest rely on surprise' is the wordplay.
'KESTREL' is hidden within the letters.
However, I'm uncertain how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for kestrel that I've seen before include "Small species of falcon" , "Bird of prey" , "It takes mice" , "Les, trek around the bird" , "High flier" .)