Sponger ruined pair's tea (8)
I believe the answer is:
parasite
'sponger' is the definition.
(thesaurus)
'ruined pair's tea' is the wordplay.
'ruined' is an anagram indicator.
'pairs'+'tea'='pairstea'
'pairstea' anagrammed gives 'PARASITE'.
(Other definitions for parasite that I've seen before include "Say mistletoe" , "one who thrives on (or off) another" , "Relies on host to survive" , "Self-interested hanger-on" , "It depends on the host" .)