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