Planet Earth's last wetland (5)
I believe the answer is:
marsh
'wetland' is the definition.
(I've seen this in another clue)
'planet earth's last' is the wordplay.
'planet' becomes 'mars' (Mars is a planet).
'last' says to take the final letters.
The last letter of 'earth' is 'h'.
'mars'+'h'='MARSH'
(Other definitions for marsh that I've seen before include "Waterlogged land" , "Bog, swamp" , "Fen" , "Swampland" , "eg Romney" .)