Island port harlot regularly visits (6)
I believe the answer is:
rialto
'island' is the definition.
(I have seen 'Island of Venice ' mean 'rialto' so perhaps 'island' could also mean 'rialto')
'port harlot regularly visits' is the wordplay.
'port' becomes 'rio' (Rio de Janeiro is a port).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'visits' indicates putting letters inside (visit can mean to go into).
The alternating letters of 'harlot' are 'alt'.
'rio' placed around 'alt' is 'RIALTO'.
(Other definitions for rialto that I've seen before include "District in Venice and in south Dublin" , "Famous Venetian bridge" , "Central area of Venice and suburb of Dublin" , "Dublin district, or Shakespeare's 'What news on the . . . . . .?'" , "Venetian district" .)