To deceive Oliver would be rash (9)
I believe the answer is:
foolhardy
'rash' is the definition.
(I know that rash can be written as foolhardy)
'to deceive oliver' is the wordplay.
'to deceive' becomes 'fool' (fooling is a kind of deceiving).
'oliver' becomes 'hardy' (I can't justify this - if you can you should believe this answer much more).
'fool'+'hardy'='FOOLHARDY'
'would be' is the link.
Can you help me to learn more?
(Other definitions for foolhardy that I've seen before include "daredevil" , "Reckless" , "Impulsive" , "Dry loofah for quality of rashness" , "Rash - incautious" .)