Bridge across one country (5)
I believe the answer is:
spain
'country' is the definition.
(I know that Spain is a country)
'bridge across one' is the wordplay.
'bridge' becomes 'span' (synonyms).
'across' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'span' enclosing 'i' is 'SPAIN'.
(Other definitions for spain that I've seen before include "part of Europe" , "Catholic country" , "European nation" , "Crazy pains?" , "European monarchy" .)