Fish in river that enters lake (6)
I believe the answer is:
tarpon
'fish' is the definition.
(I've seen this before)
'river that enters lake' is the wordplay.
'river' becomes 'po' (Italian river).
'that enters' means one lot of letters goes inside another.
'lake' becomes 'tarn' (tarn is a kind of lake).
'po' going inside 'tarn' is 'TARPON'.
'in' is the link.
(Other definitions for tarpon that I've seen before include "Large silvery game fish of coastal waters off Florida" , "Great fish of Florida" , "No part for this large fish" .)