Cathedral pressure over what bishop wears in rich supply (10)
I believe the answer is:
wellspring
'rich supply' is the definition.
(a wellspring is a plentiful supply of something)
'cathedral pressure over what bishop wears' is the wordplay.
'cathedral' becomes 'Wells' (Wells Cathedral in Somerset).
'pressure' becomes 'p' (abbreviation).
'over' is a charade indicator (letters next to each other).
'what bishop wears' becomes 'ring' (ceremonial ring).
'wells'+'p'+'ring'='WELLSPRING'
'in' is the link.
(Other definitions for wellspring that I've seen before include "Rich provider" , "Plentiful supply" , "Abundant source" .)