Broken ribs on birds (6)
I believe the answer is:
robins
'birds' is the definition.
(robin is a kind of bird)
'broken ribs on' is the wordplay.
'broken' indicates anagramming the letters.
'ribs'+'on'='ribson'
'ribson' is an anagram of 'ROBINS'.
(Other definitions for robins that I've seen before include "Small songbirds" , "Friendly winter birds" , "Small thrushes" , "Birds with red breasts" , "Birds with reddish breasts" .)