A priest is disturbed about a sponger (8)
I believe the answer is:
parasite
'sponger' is the definition.
(I know that sponger can be written as parasite)
'a priest is disturbed about a' is the wordplay.
'is disturbed' indicates an anagram (the order of letters is disturbed).
'about' means one lot of letters goes inside another.
'a'+'priest'='apriest'
'apriest' anagrammed gives 'parsite'.
'parsite' enclosing 'a' is 'PARASITE'.
(Other definitions for parasite that I've seen before include "mistletoe sprig can be" , "Relies on host to survive" , "it sucks blood" , "It lives off its host" , "unwanted guest" .)