Would you put your faith in a builder who could do so? (7)
I believe the answer is:
convert
'would you put your faith in a builder who' is the definition.
I can't judge whether this defines the answer.
'do so?' is the wordplay.
'do' becomes 'con' (both can mean to defraud).
'so?' becomes 'vert' (I am not sure about this - if you are sure you should believe this answer much more).
'con'+'vert'='CONVERT'
'could' acts as a link.
Can you help me to learn more?
(Other definitions for convert that I've seen before include "Transform" , "Win over opponent" , "succeed at Twickenham" , "Turn" , "new enthusiastic believer?" .)