Repent at leisure if you act thus, proverbially (2,5)
I believe the answer is:
in haste
'proverbially' is the definition.
Both the definition and answer are adverbs. Perhaps there's a link between them I don't understand?
'repent at leisure if you act thus' is the wordplay.
I cannot really see how this works, but
'at' could be 'in' and 'in' is found within the answer.
'act' could be 'haste' (haste is a kind of act) and 'haste' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for in haste that I've seen before include "''Marry . . . . . . ., repent at leisure'' (2, 5)" , "Quickly" , "Rushing round" , "Rapidly" , "Hurriedly" .)