Spoil the planet, endlessly (3)
I believe the answer is:
mar
'spoil' is the definition.
('mar' can be a synonym of 'spoil')
'the planet endlessly' is the wordplay.
'the planet' becomes 'mars' (Mars is an example).
'endlessly' means to remove the last letter.
'mars' with its final letter taken off is 'MAR'.
(Other definitions for mar that I've seen before include "A windy month" , "Ruin" , "Blight" , "Blemish" , "hurt" .)