Adonis lowers bar (8)
I believe the answer is:
beefcake
'adonis' is the definition.
'lowers bar' is the wordplay.
'lowers' becomes 'beef' (cattle might 'low' - another word for moo).
'bar' becomes 'cake' ('cake' can be a synonym of 'bar').
'beef'+'cake'='BEEFCAKE'
(Other definitions for beefcake that I've seen before include "musclemen" , "Displayed male muscles" , "Certain photos" , "Strong talent" , "Hunk" .)