Holiest city of Islam (5)
I believe the answer is:
mecca
'holiest city of islam' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mecca that I've seen before include "Tourist magnet" , "place with devoted visitors" , "Holiest city of 13 Across, place of annual pilgrimage" , "Islam's holiest city" , "hallowed ground" .)