Reckon US author is to marry (7)
I believe the answer is:
betroth
'to marry' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how one could define the other.
'reckon us author' is the wordplay.
'reckon' becomes 'bet' (bet can mean to assume or reckon something is true).
'us author' becomes 'Roth' (novelist Philip Roth).
'bet'+'roth'='BETROTH'
'is' is the link.
Can you help me to learn more?
(Other definitions for betroth that I've seen before include "Promise to match" , "Promise to marry" , "Affiance (one person to another)" , "contract" , "Engage to be married" .)