You and I, having penned nonsense, were authors (5)
I believe the answer is:
wrote
'were authors' is the definition.
(if they wrote things, they were authors)
'you and i having penned nonsense' is the wordplay.
'you and i' becomes 'we'.
'having penned' is an insertion indicator (inserted letters are enclosed or penned in).
'nonsense' becomes 'rot' (rot can mean nonsense or rubbish).
'we' enclosing 'rot' is 'WROTE'.
(Other definitions for wrote that I've seen before include "Put to paper" , "Composed (music)" , "Inscribed" , "Penned" .)