Spiritual leader, one attending mass before noon (4)
I believe the answer is:
imam
'spiritual leader' is the definition.
(prayer leader in Islam)
'one attending mass before noon' is the wordplay.
'one' becomes 'i' (roman numeral).
'attending' means one lot of letters go next to another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'before noon' becomes 'am'.
'i'+'m'+'am'='IMAM'
(Other definitions for imam that I've seen before include "Muslim preacher" , "Authority on Islamic doctrine and law" , "Muslim religious leader" , "Man leading prayers in a mosque" , "Prayer-leader in a mosque" .)