Clergyman swallows one: very health-giving (8)
I believe the answer is:
curative
'health-giving' is the definition.
(I've seen this before)
'clergyman swallows one very' is the wordplay.
'clergyman' becomes 'curate' (curate is a kind of clergyman).
'swallows' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'very' becomes 'v' (abbreviation).
'i'+'v'='iv'
'curate' enclosing 'iv' is 'CURATIVE'.
(Other definitions for curative that I've seen before include "health-giving" , "Therapeutic" , "Restoring to health" , "Medicine" , "Healing" .)