Tower on a mosque (7)
I believe the answer is:
minaret
'tower on a mosque' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for minaret that I've seen before include "Slim tower of a mosque" , "Slender mosque tower" , "Slender tower with balconies" , "tower in Istanbul, say" , "Tower from which muezzin calls Muslims to prayer" .)