Religious leader is a single mother (4)
I believe the answer is:
imam
'religious leader' is the definition.
(I know that imam is a type of religious leader)
'single mother' is the wordplay.
'single' becomes 'I' (one).
'mother' becomes 'mam' (I've seen this in other clues).
'i'+'mam'='IMAM'
'is a' is the link.
(Other definitions for imam that I've seen before include "Muslim community leader" , "Prayer leader in a mosque" , "Man leading prayers in a mosque" , "Islamic minister" , "Muslim preacher" .)