Faith on island weak for the most part (5)
I believe the answer is:
islam
'faith' is the definition.
(I know that Islam is a type of monotheism)
'on island weak for the most part' is the wordplay.
'on' says to put letters next to each other.
'island' becomes 'is' (abbreviation).
'weak' becomes 'lame' (similar in meaning).
'for the most part' means to remove the last letter (most but not all of the letters).
'lame' with its last letter removed is 'lam'.
'is' next to 'lam' is 'ISLAM'.
(Other definitions for islam that I've seen before include "Mails to Muslim religion" , "Religion of the Koran" , "Belief" , "Religion based on teachings of Muhammad" , "Mails to great eastern religion" .)