Continent oddly not one seen in map collection! (8)
I believe the answer is:
atlantis
'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'.
'nt'+'i'='nti'
'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" .)