Continent oddly not one seen in map collection! (8)
I believe the answer is:
'continent' is the definition.
(I've seen this before)
'oddly not one seen in map collection' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'one' becomes 'i' (Roman numeral).
'seen in' is an insertion indicator.
'map collection' becomes 'atlas' (synonyms).
The alternate letters of 'not' are 'nt'.
'nti' placed within 'atlas' is 'ATLANTIS'.
(Other definitions for atlantis that I've seen before include "Fabled sunken island" , "In Greek legend, beautiful island lost in the sea" , "Fabled lost island" , "Mythical city under sea" , "Legendary lost continent" .)