Mum got separated visiting an Italian island, which leaves a bitter taste (7)
I believe the answer is:
campari
'which leaves a bitter taste' is the definition.
(bitter drink)
I don't understand how the remainder of the clue works.
(Other definitions for campari that I've seen before include "Mildly bitter aperitif, often drunk with soda" , "bitter drink" , "Pinkish aperitif" , "Red, bitter-sweet Italian aperitif" .)