Ready to toy with carelessly, my one and only old mop (8,5)
I believe the answer is:
monopoly money
'ready to toy with' is the definition.
(cash or 'ready' in the game Monopoly)
'carelessly my one and only old mop' is the wordplay.
'carelessly' indicates anagramming the letters.
'and' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'my'+'one'+'only'+'o'+'mop'='myoneonlyomop'
'myoneonlyomop' anagrammed gives 'MONOPOLY MONEY'.
(Other definitions for monopoly money that I've seen before include "Pretend cash" , "Ready for play" , "Large sums treated lightly" , "Large sums regarded as valueless" , "Large sums that are virtually worthless" .)