Profound philosopher's first seen by river (4)
I believe the answer is:
deep
'profound' is the definition.
(synonyms)
'philosopher's first seen by river' is the wordplay.
'philosopher's first' becomes 'p' (dst letter of 'philosopher').
'seen by' says to put letters next to each other.
'river' becomes 'dee' (River Dee).
'p' after 'dee' is 'DEEP'.
(Other definitions for deep that I've seen before include "hard to follow" , "hard to understand" , "Profound - cunning - sea - bass" , "Intense; low (in pitch)" , "Extending far down" .)