Defiant expression from boorish lout around bar's entrance (3-3)

'defiant expression' is the definition.
('yah-boo' might be said in defiance or rudeness)

'boorish lout around bar's entrance' is the wordplay.
'boorish lout' becomes 'yahoo' (yahoo can mean a loutish person).
'around' is an insertion indicator.
'entrance' suggests taking the first letters.
The initial letter of 'bars' is 'b'.
'yahoo' enclosing 'b' is 'YAH-BOO'.

'from' is the link.