Wet planet before humanity ends (6)
I believe the answer is:
marshy
'wet' is the definition.
('marshy' can be similar in meaning to 'wet')
'planet before humanity ends' is the wordplay.
'planet' becomes 'mars' (Mars is an example).
'before' is a charade indicator (letters next to each other).
'ends' says to hollow out the word (remove centre letters).
'humanity' with its middle removed is 'hy'.
'mars'+'hy'='MARSHY'
(Other definitions for marshy that I've seen before include "Waterlogged ground" , "Not solidly grounded" , "(Of soil) soft and wet" , "Damp and low-lying" , "(Of land) waterlogged" .)