A planet's teribly nice (8)
I believe the answer is:
pleasant
'nice' is the definition.
(associated in meaning)
'a planet's teribly' is the wordplay.
'a planet's' can be anagrammed to 'PLEASANT'.
But, I'm not clear how the anagram is indicated.
Can you help me to learn more?
(Other definitions for pleasant that I've seen before include "Satisfying, likeable" , "Nice, satisfying" , "Enjoyable, nice" , "Attractive" , "sweet?" .)