Revolutionary, greeting regularly that year foreign cleric (9)
I believe the answer is:
ayatollah
'foreign cleric' is the definition.
I know nothing about this answer so I can't tell whether this works.
'revolutionary greeting regularly that year' is the wordplay.
'revolutionary' says the letters should be written backwards (letters revolve around).
'greeting' becomes 'hallo' (alternative to hello).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'that'+'year'='thatyear'
The alternate letters of 'thatyear' are 'taya'.
'hallo'+'taya'='hallotaya'
'hallotaya' in reverse letter order is 'AYATOLLAH'.
Can you help me to learn more?
(Other definitions for ayatollah that I've seen before include "High-ranking Shiite religious leader" , "Religious high-up" , "Shiite leader, possibly in 10 across" , "Iranian leader" , "religious authority" .)