In the Muslim world, one encounters something possibly grand (5)
I believe the answer is:
islam
'in the muslim world one encounters something' is the definition.
I can't judge whether this definition defines the answer.
'possibly grand' is the wordplay.
'possibly' becomes 'isla' (I can't justify this - if you can you should believe this answer much more).
'grand' becomes 'M' (synonyms).
'isla'+'m'='ISLAM'
Can you help me to learn more?
(Other definitions for islam that I've seen before include "Religion based on teachings of Muhammad" , "Koran its scripture" , "Mohammed's religion" , "Religion of the Koran" , "Belief" .)