Cardinal visiting London in Ealing (4)
I believe the answer is:
nine
'cardinal' is the definition.
(a cardinal number)
'visiting london in ealing' is the wordplay.
'visiting' indicates the answer is hidden within the clue (the answer 'visits' the inside of the clue).
'NINE' is hidden in the letters of 'london in ealing'.
(Other definitions for nine that I've seen before include "Number of feline existences" , "Three squared" , "Figure" , "It's dialled three times in an emergency" , "'Possession is... points of the law'" .)