Beat up fat bird (7)
I believe the answer is:
mallard
'bird' is the definition.
(mallard is a kind of bird)
'beat up fat' is the wordplay.
'beat' becomes 'lam' (I've seen this before).
'up' is a reversal indicator (in down clue: letters go upwards).
'fat' becomes 'lard' (I've seen this before).
'lam' back-to-front is 'mal'.
'mal'+'lard'='MALLARD'
(Other definitions for mallard that I've seen before include "Variety of duck" , "web-footed bird" , "The famous steam train" , "Ancestor of all domestic ducks" , "Oldest breed of duck" .)