A German returns to cross lake and river (4)
I believe the answer is:
nile
'river' is the definition.
(Nile River is an example)
'a german returns to cross lake' is the wordplay.
'a german' becomes 'ein' ('a' in German).
'returns' is a reversal indicator.
'to cross' means one lot of letters goes inside another.
'lake' becomes 'l'.
'ein' backwards is 'nie'.
'nie' placed around 'l' is 'NILE'.
'and' acts as a link.
(Other definitions for nile that I've seen before include "River that's Blue and White" , "River at Luxor" , "'The longest river, blue and white (4)'" , "Line (anag)" , "River of Egypt" .)