Regret finding European fool in the money (9)
I believe the answer is:
penitence
'regret' is the definition.
(I know that penitence is a type of remorse)
'european fool in the money' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'fool' becomes 'nit' (both can mean a stupid person).
'in the' means one lot of letters goes inside another.
'money' becomes 'pence' (I've seen this before).
'e'+'nit'='enit'
'enit' placed within 'pence' is 'PENITENCE'.
'finding' acts as a link.
(Other definitions for penitence that I've seen before include "Contriteness" , "shame" , "A show of regret for wrongdoing" , "Contrition, remorse" , "display of remorse" .)