Jellyfish in sea to America's north? (6)
I believe the answer is:
medusa
'jellyfish' is the definition.
(synonyms)
'in sea to america's north?' is the wordplay.
'in' is an insertion indicator.
'sea' becomes 'med' (abbreviation of Mediterranean).
'to america' becomes 'a' (common abbreviation - e.g. in organisation names).
'north?' becomes 'US' (north is a kind of U.S.).
'med'+'a'='meda'
'meda' going around 'us' is 'MEDUSA'.
(Other definitions for medusa that I've seen before include "Snake-haired gorgon" , "Amused like Greek Gorgon" , "Amused by the mythical gorgon?" , "Mythological gorgon" , "Hideous female" .)