Bird that is an awful brawler (7)
I believe the answer is:
warbler
'bird' is the definition.
(warbler is a kind of bird)
'an awful brawler' is the wordplay.
'an awful' indicates an anagram.
'brawler' anagrammed gives 'WARBLER'.
'that is' acts as a link.
(Other definitions for warbler that I've seen before include "18's [BIRD]" , "XWD SETTER" , "songster" , "A small songbird or similar singer" .)