Muslim star going over part of UK (5)
I believe the answer is:
'muslim' is the definition.
(Sunni is an example)
'star going over part of uk' is the wordplay.
'star' becomes 'sun' (Sun is an example).
'going over' means one lot of letters go next to another.
'part of uk' becomes 'ni' (Northern Ireland).
(Other definitions for sunni that I've seen before include "A main branch of Islam" , "Indian fig tree with aerial roots" , "Moslem" , "One branch of Islam" , "Islamic faction" .)