Mother! I am about to be a mosque leader (4)
I believe the answer is:
imam
'a mosque leader' is the definition.
(I've seen this before)
'mother i am about' is the wordplay.
'mother' becomes 'mam' (I've seen this in other clues).
'i am' becomes 'i' (I've seen this in other clues).
'about' says the letters should be written in reverse.
'mam'+'i'='mami'
'mami' written backwards gives 'IMAM'.
'to be' is the link.
(Other definitions for imam that I've seen before include "Authority on Islamic doctrine and law" , "Muslim holy man" , "Islamic minister" , "Islamic prayer leader" , "Maim (anag)" .)