I'm not sure Mother has joined the Muslim community (4)
I believe the answer is:
umma
'muslim community' is the definition.
(I've seen this before)
'i'm not sure mother has joined' is the wordplay.
I cannot quite understand how this works, but
'mother' could be 'ma' (ma can mean mother) and 'ma' is found within the answer.
'not sure' could be 'um' (both might be said when unsure about something) and 'um' is located in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
'the' acts as a link.
(Other definitions for umma that I've seen before include "Muslim body" , "Muslim community" .)