Regret informal night surrounded by money (9)
I believe the answer is:
penitence
'regret' is the definition.
(penitence is a kind of regret)
'informal night surrounded by money' is the wordplay.
'informal night' becomes 'nite' (I can't explain this - if you can you should believe this answer much more).
'surrounded by' means one lot of letters goes inside another.
'money' becomes 'pence ' (I've seen this before).
'nite' going into 'pence' is 'PENITENCE'.
Can you help me to learn more?
(Other definitions for penitence that I've seen before include "Contrition, remorse" , "display of remorse" , "A show of regret for wrongdoing" , "shame" , "Contriteness" .)