Be a socially acceptable lover (4)
I believe the answer is:
'lover' is the definition.
(I know that beau is a type of lover)
'be a socially acceptable' is the wordplay.
'socially acceptable' becomes 'u'.
(Other definitions for beau that I've seen before include "Male admirer; dandy" , "An old-fashioned boy friend (4)" , "Sweetheart" , "Dandy - attentive companion" , "Handsome man" .)