Country under Portugal has endless fish! (7)
I believe the answer is:
piranha
'fish' is the definition.
(piranha is a kind of fish)
'country under portugal has endless' is the wordplay.
'country' becomes 'iran' (Iran is a country).
'under' means one lot of letters go next to another (in a down clue, some letters go below others).
'portugal' becomes 'p' (common abbreviation eg on car number plates).
'endless' means to remove the last letter.
'has' with its last letter removed is 'ha'.
'iran' put after 'p' is 'piran'.
'piran'+'ha'='PIRANHA'
(Other definitions for piranha that I've seen before include "fish chewed up" , "Small carnivorous freshwater fish" , "Ferocious fish" , "Small voracious carnivorous fish" , "This predator" .)