Existing expenditure (4,2,6)
I believe the answer is:
cost of living
'expenditure' is the definition.
(cost of living is a kind of expenditure)
'existing' is the wordplay.
I cannot really understand how this works, but
'existing' could be 'living' ('live' can be a synonym of 'exist') and 'living' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for cost of living that I've seen before include "What the RPI and CPI monitor" , "Level of everyday prices" .)