Lower centre of dartboard (4)
I believe the answer is:
bull
I believe this is a double definition.
'lower' is the first definition.
(animal that lows - low means to moo)
'centre of dartboard' is the second definition.
(bullseye)
(Other definitions for bull that I've seen before include "Bovine animal" , "Male animal" , "Male cow" , "Male elephant, say" , "See 20 down" .)