One in a rush to claim riches involved demon girl (4,5)
'one in a rush to claim riches' is the definition.
(person who mines for riches)
'involved demon girl' is the wordplay.
'involved' is an anagram indicator (involved can mean confusing or complex).
'demongirl' anagrammed gives 'GOLD MINER'.
(Another definition for gold miner that I've seen is "fortune hunter".)