First of parr following river leading to ocean (4)
I believe the answer is:
deep
'ocean' is the definition.
(thesaurus)
'first of parr following river' is the wordplay.
'first of' indicates taking the first letters.
'following' is a charade indicator (letters next to each other).
'river' becomes 'dee' (River Dee**).
The first letter of 'parr' is 'p'.
'p' put after 'dee' is 'DEEP'.
'leading to' is the link.
(Other definitions for deep that I've seen before include "greatly involved?" , "Extending far down" , "Sunk low" , "hard to understand" , "Intense -- extreme" .)