Abundantly rocky planet, last in galaxy (7)
I believe the answer is:
aplenty
'abundantly' is the definition.
(thesaurus)
'rocky planet last in galaxy' is the wordplay.
'rocky' indicates anagramming the letters.
'last in' says to take the final letters.
The last letter of 'galaxy' is 'y'.
'planet' with letters rearranged gives 'aplent'.
'aplent'+'y'='APLENTY'
(Other definitions for aplenty that I've seen before include "heaps" , "lots of it" , "In abundance" , "more than enough" , "by the score" .)