Round closing time, Arkwright's not -- we trust -- purveying pork pies! (2,4)
I believe the answer is:
on oath
'not we trust purveying pork pies' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'round closing time arkwright's' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'closing' indicates putting letters inside ('closing' can also mean 'enclosing').
'time' becomes 't' (abbreviation).
'arkwright' becomes 'noah' (I've seen this before).
't' going inside 'noah' is 'noath'.
'o'+'noath'='ON OATH'
Can you help me to learn more?
(Other definitions for on oath that I've seen before include "Bound to speak the truth" , "Sworn to speak the truth" .)