Key old rival wasted money (7)
I believe the answer is:
bolivar
'key old rival wasted money' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'key old rival wasted' is the wordplay.
'key' becomes 'b' (eg B major, musical key).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'wasted' is an anagram indicator.
'rival' with letters rearranged gives 'livar'.
'b'+'o'+'livar'='BOLIVAR'
Can you help me to learn more?
(Other definitions for bolivar that I've seen before include "Foreign brass" , "Leader of several independence movements by Spain's South American colonies, d. 1830" , "Currency of Venezuela" , "Simon --, South American liberator" , "Founder of Bolivia" .)