One book elevated Arab - that chap’s a key Muslim figure (7)
I believe the answer is:
ibrahim
'a key muslim figure' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'one book elevated arab that chap's' is the wordplay.
'one' becomes 'i' (Roman numeral).
'book' becomes 'b' (abbreviation for book).
'elevated' shows that the letters should be reversed in order (in a down clue, letters go up).
'arab' becomes 'Ar.' (abbreviation in reference works).
'that chap' becomes 'him' (I've seen this in another clue).
'ar' back-to-front is 'ra'.
'i'+'b'+'ra'+'him'='IBRAHIM'
Can you help me to learn more?
(Other definitions for ibrahim that I've seen before include "Islamic prophet" , "Moslem" .)