1,000 in currency is 500 in sterling (6)
I believe the answer is:
monkey
'500 in sterling' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'1000 in currency' is the wordplay.
'1000' becomes 'k' ('K' can be a synonym of '1000').
'in' is an insertion indicator.
'currency' becomes 'money' (money is a kind of currency).
'k' inserted inside 'money' is 'MONKEY'.
'is' is the link.
Can you help me to learn more?
(Other definitions for monkey that I've seen before include "Long-tailed primate" , "Naughty child; primate" , "Small primate" , "Mischievous child; primate" , "PS500 (sl.)" .)