Wager? Rubbish! Harold has an engagement (9)
I believe the answer is:
betrothal
'an engagement' is the definition.
(I know that engagement can be written as betrothal)
'wager? rubbish harold' is the wordplay.
'wager?' becomes 'bet' ('bet' can be a synonym of 'wager').
'rubbish' becomes 'rot' (both can mean nonsense).
'harold' becomes 'hal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bet'+'rot'+'hal'='BETROTHAL'
'has' acts as a link.
Can you help me to learn more?
(Other definitions for betrothal that I've seen before include "Engagement to marry" , "Pledge" , "Plight" , "Promise of marriage" , "Commitment" .)