Muslim religious leader (9)
'muslim religious leader' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for imam that I've seen before include "Caliph" , "Muslim religious leader" , "Person leading devotions in a mosque" , "Strict religious leader" , "Accompaniment in baroque music" .)
(Other definitions for ayatollah that I've seen before include "Muslim leader" , "religious authority" , "Shiite leader, possibly in 10 across" , "Iranian leader" , "Senior Iranian cleric" .)