Some kangaroos terrifying fowl (7)
I believe the answer is:
rooster
'fowl' is the definition.
(I know that rooster is a type of Gallus gallus)
'some kangaroos terrifying' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'ROOSTER' is hidden in the letters of 'kangaroos terrifying'.
(Other definitions for rooster that I've seen before include "Farmyard fowl" , "one going to sleep" , "Bird that wakes up" , "Farmyard bird" , "Early-morning caller" .)