This person's with mother, looking up to religious leader (4)
I believe the answer is:
imam
'to religious leader' is the definition.
(I've seen this before)
'this person's with mother looking up' is the wordplay.
'this person's' becomes 'I'm' (('I am' - 'this person' means the crossword setter).
'with' means one lot of letters go next to another.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'looking up' says the letters should be written in reverse (in a down clue, letters go up).
'ma' backwards is 'am'.
'im'+'am'='IMAM'
(Other definitions for imam that I've seen before include "Man leading prayers in a mosque" , "Person leading devotions in a mosque" , "Strict religious leader" , "Caliph" , "Moslem prayer leader" .)