A large lake going up country (4)
I believe the answer is:
eire
'country' is the definition.
(Eire is an example)
'a large lake going up' is the wordplay.
'a large lake' becomes 'erie' (one of the Great Lakes).
'going up' shows that the letters should be reversed in order.
'erie' in reverse letter order is 'EIRE'.
(Other definitions for eire that I've seen before include "island" , "Breakaway state" , "Gaelic name for Republic of Ireland" , "EU country" , "Look for (something to happen)" .)