Fine planet crumbling, ending in decay (7)
I believe the answer is:
penalty
'fine' is the definition.
(thesaurus)
'planet crumbling ending in decay' is the wordplay.
'crumbling' indicates anagramming the letters.
'ending in' says to take the final letters.
The final letter of 'decay' is 'y'.
'planet' anagrammed gives 'penalt'.
'penalt'+'y'='PENALTY'
(Other definitions for penalty that I've seen before include "Fine - handicap" , "Punishment eg for foul" , "Perhaps fine" , "Punishment for error (in game)" , "Disadvantage imposed" .)