Ruin nearly all the planet (3)
I believe the answer is:
mar
'ruin' is the definition.
(I've seen this before)
'nearly all the planet' is the wordplay.
'nearly all' means to remove the last letter.
'the planet' becomes 'mars' (Mars is an example).
'mars' with its final letter taken away is 'MAR'.
(Other definitions for mar that I've seen before include "Detract from" , "Spoil stuff" , "wreck" , "Blemish" , "Spoil, damage" .)