Uncle swimming across English river and lake (7)
I believe the answer is:
lucerne
'lake' is the definition.
(Swiss lake)
'uncle swimming across english river' is the wordplay.
'swimming' indicates an anagram.
'across' indicates putting letters inside.
'english' becomes 'e' (abbreviation).
'river' becomes 'r'.
'uncle' with letters rearranged gives 'lucne'.
'e'+'r'='er'
'lucne' placed around 'er' is 'LUCERNE'.
'and' acts as a link.
(Other definitions for lucerne that I've seen before include "Swiss city - alfalfa" , "Source of chlorophyll" , "Swiss city; cattle feed" , "Swiss city/lake" , "in Switzerland" .)