Main country for monster (6)
I believe the answer is:
medusa
'for monster' is the definition.
(Medusa is an example)
'main country' is the wordplay.
'main' becomes 'med' ('main' can mean the sea - eg Mediterranean or Med).
'country' becomes 'usa' (U.S.A. is an example).
'med'+'usa'='MEDUSA'
(Other definitions for medusa that I've seen before include "Gorgon of Greek myth" , "Gorgon with snakes for hair" , "Snake-haired gorgon" , "Gorgon killed by Perseus - jellyfish" , "Petrifying woman" .)