One using ultrasound for detecting speed (3)
I believe the answer is:
bat
I believe this is a double definition.
'one using ultrasound for detecting' is the first definition.
'speed' is the second definition.
(thesaurus)
(Other definitions for bat that I've seen before include "Flutter (eyelids)" , "Small flying mammal - like 11 Across" , "A mammal; a club" , "Night-flyer" , "Night-flying mammal" .)