Parasite finds shelter on top of church (5)
I believe the answer is:
leech
'parasite' is the definition.
(synonyms)
'shelter on top of church' is the wordplay.
'shelter' becomes 'lee' (as in lee side, the side sheltered from wind).
'on top of' is a charade indicator (letters next to each other).
'church' becomes 'ch' (common abbreviation).
'lee'+'ch'='LEECH'
'finds' is the link.
(Other definitions for leech that I've seen before include "Medicinal bloodsucker" , "and one of his remedies" , "Proverbial bloodsucker" , "Bloodsucker; early doctor" , "One who sponges on others" .)