Profound river pilot's leader (4)
I believe the answer is:
deep
'profound' is the definition.
('deep' can be a synonym of 'profound')
'river pilot's leader' is the wordplay.
'river' becomes 'dee' (River Dee**).
'leader' suggests taking the first letters.
The first letter of 'pilots' is 'p'.
'dee'+'p'='DEEP'
(Other definitions for deep that I've seen before include "Intense -- extreme" , "Sunk low" , "Bottomless" , "late" , "Cavernous" .)