Crumbling older reef hosting a parasite (10)
I believe the answer is:
freeloader
'parasite' is the definition.
(I've seen this in another clue)
'crumbling older reef hosting a' is the wordplay.
'crumbling' indicates an anagram.
'hosting' indicates putting letters inside.
'older'+'reef'='olderreef'
'olderreef' anagrammed gives 'freeloder'.
'freeloder' enclosing 'a' is 'FREELOADER'.
(Other definitions for freeloader that I've seen before include "Sponger (informal)" , "One who takes but never gives" , "One who gains from others' efforts" , "One living off others" , "I'm a sucker" .)