Bitter about a rocker's rival being fashionable (1,2,4)
I believe the answer is:
a la mode
'fashionable' is the definition.
(a la mode can mean trendy or fashionable)
'bitter about a rocker's rival' is the wordplay.
'bitter' becomes 'ale' (type of beer or bitter).
'about' is an insertion indicator.
'rocker's rival' becomes 'mod' (I've seen this in another clue).
'a'+'mod'='amod'
'ale' enclosing 'amod' is 'A-LA-MODE'.
'being' is the link.
(Other definitions for a la mode that I've seen before include "stewed with veg?" , "With ice cream on top or by the side" , "Fashionable (French)" , "In the fashion, in Paris?" , "Do a meal in French style (1, 2, 4)" .)