Adonis lowers bar (8)
I believe the answer is:
Here is my best explanation:
'adonis' is the definition.
'lowers bar' is the subsidiary indication.
'lowers' becomes 'beef' (cattle might 'low' - another word for moo).
'bar' becomes 'cake' ('cake' can be a synonym of 'bar').
(Other definitions for beefcake that I've seen before include "musclemen", "Men displaying muscles", "Strong talent", "Photo of a muscular man with not much on", "Hunk", "Certain photos", "Very muscular men", "Displayed male muscles", "Attractive muscular men" and "Muscular attractive men".)