Mammal visits old church a great deal (6)
I believe the answer is:
ocelot
'mammal' is the definition.
(ocelot is a kind of mammal)
'old church a great deal' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'church' becomes 'CE' (abbreviation for Church of England).
'a great deal' becomes 'lot' (synonyms).
'o'+'ce'+'lot'='OCELOT'
'visits' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ocelot that I've seen before include "Type of American wildcat" , "Striped and spotted wildcat" , "Animal in the loo etc." , "Wild creature" , "Fur" .)