Belligerent seabird smaller ones upset (4)
I believe the answer is:
skua
This could be a double definition.
'belligerent seabird' is the first definition.
'skua' can be an answer for 'seabird' (type of seabird). I'm not certain of the 'belligerent' bit.
'smaller ones upset' is the second definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
Can you help me to learn more?
(Other definitions for skua that I've seen before include "Large predatory bird" , "Flyer" , "Large relative of the gull" , "Craziness" , "Seabird; sounds like pin" .)