Perhaps sister is visiting European Union (3)
I believe the answer is:
nun
'perhaps sister' is the definition.
(I know that sister is a type of Christian nun)
'visiting european union' is the wordplay.
'visiting' says the answer is hidden in the clue (the answer 'visits' the inside of the clue).
'NUN' is hidden within 'european union'.
'is' is the link.
(Other definitions for nun that I've seen before include "Mother Teresa, maybe" , "Carmelite, for example" , "Holy sister" , "Female in religious order" , "Religious woman, a palindrome" .)