National Trust island in collection of maps is long-lost land (8)
I believe the answer is:
atlantis
'lost land' is the definition.
(I've seen this before)
'national trust island in collection of maps' is the wordplay.
'national trust' becomes 'nt'.
'island' becomes 'i' (geographical abbreviation).
'in' indicates putting letters inside.
'collection of maps' becomes 'atlas' (I've seen this before).
'nt'+'i'='nti'
'nti' placed within 'atlas' is 'ATLANTIS'.
'is long' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for atlantis that I've seen before include "Legendary kingdom overwhelmed by the sea" , "Legendary island - in at last (anag)" , "In legend, a continent under the sea" , "Legendary long-lost island" , "Lost island of Greek myth" .)