Land one left to enter river (5)
I believe the answer is:
italy
'land' is the definition.
(Italy is an example)
'one left to enter river' is the wordplay.
'one' becomes 'I' (Roman numeral).
'left' becomes 'l' (common abbreviation).
'to enter' is an insertion indicator.
'river' becomes 'tay' (Scottish river).
'l' going into 'tay' is 'taly'.
'i'+'taly'='ITALY'
(Other definitions for italy that I've seen before include "Mediterranean country" , "Problematical (informal)" , "Catholic country" , "South European country" , "Land of Tuscany and Umbria" .)