River Po's opening leading to sea (4)
I believe the answer is:
deep
'sea' is the definition.
(thesaurus)
'river po's opening leading' is the wordplay.
'river' becomes 'dee' (River Dee**).
'opening' suggests taking the first letters.
'leading' means one lot of letters go next to another.
The initial letter of 'pos' is 'p'.
'dee'+'p'='DEEP'
'to' acts as a link.
(Other definitions for deep that I've seen before include "late" , "Extending far down" , "Intense; low (in pitch)" , "Main" , "Sunk low" .)