A converted vicar promises to be greedy (10)
I believe the answer is:
avaricious
'greedy' is the definition.
(synonyms)
'a converted vicar promises' is the wordplay.
'converted' indicates anagramming the letters.
'promises' becomes 'IOUs' (promises to pay a debt).
'vicar' anagrammed gives 'varic'.
'a'+'varic'+'ious'='AVARICIOUS'
'to be' is the link.
(Other definitions for avaricious that I've seen before include "Greedy" , "mercenary" , "keen to grab treasure?" .)