Wishful thinker that is accepting Muhammad, say (8)
I believe the answer is:
idealist
'wishful thinker' is the definition.
(I've seen this before)
'accepting muhammad say' is the wordplay.
'accepting' is an insertion indicator (inserted letters are accepted).
'muhammad' becomes 'ali' (I've seen this in another clue).
'say' becomes 'idest' (I can't justify this - if you can you should give a lot more credence to this answer).
'ali' placed within 'idest' is 'IDEALIST'.
'that is' acts as a link.
Can you help me to learn more?
(Other definitions for idealist that I've seen before include "Sail tide (anag.)" , "Romantic sort" , "Detail is about person with aspirations toward perfection (8)" , "Perfectionist" , "One who aspires to a perfect world" .)