My one lost cash (5)
I believe the answer is:
money
'cash' is the definition.
(thesaurus)
'my one lost' is the wordplay.
'lost' is an anagram indicator.
'my'+'one'='myone'
'myone' with letters rearranged gives 'MONEY'.
(Other definitions for money that I've seen before include "It's the root of all evil" , "Euro for example" , "See 7" , "tin" , "Lucre" .)