Jeopardise mischievous child by sending over old money (7)
I believe the answer is:
imperil
'jeopardise' is the definition.
(synonyms)
'mischievous child by sending over old money' is the wordplay.
'mischievous child' becomes 'imp' (imp can mean a misbehaving child).
'by' is a charade indicator (letters next to each other).
'sending over' is a reversal indicator.
'old money' becomes 'lire' (plural of lira, former currency of Italy).
'lire' reversed gives 'eril'.
'imp'+'eril'='IMPERIL'
(Other definitions for imperil that I've seen before include "Endanger" , "Jeopardize" , "Place in danger" , "Expose" , "Put at risk" .)