Clairvoyant visited by Saint David, most famously (8)
I believe the answer is:
psalmist
'david most famously' is the definition.
(author of many of the biblical Psalms)
'clairvoyant visited by saint' is the wordplay.
'clairvoyant' becomes 'palmist' (palm reader).
'visited by' is an insertion indicator.
'saint' becomes 's'.
'palmist' going around 's' is 'PSALMIST'.
(Other definitions for psalmist that I've seen before include "poetic king ..." , "Writer of sacred songs in O.T." , "Singer of sacred songs" , "Old religious writer" , "King David" .)