This must endlessly rebuild some land (7)
I believe the answer is:
isthmus
'some land' is the definition.
(isthmus is a kind of land)
'this must endlessly rebuild' is the wordplay.
'endlessly' means to remove the last letter.
'rebuild' indicates anagramming the letters (I've seen 'rebuilding' mean this).
'this'+'must'='thismust'
'thismust' with its final letter removed is 'thismus'.
'thismus' with letters rearranged gives 'ISTHMUS'.
(Other definitions for isthmus that I've seen before include "Narrow strip of land connecting two large masses" , "Narrow stretch of land joining two larger masses" , "eg Corinth, Panama" , "This sum (anag)" , "Connective strip of land" .)