Crossword Genius

Service charges for castles in the air? (6)

Ross

I believe the answer is:

upkeep

I'm a little stuck... Click here to teach me more about this clue!

'service' is the definition.

'charges for castles in the air?' is the wordplay.
I cannot really understand how this works, but
'in the air?' could be 'up' and 'up' is located in the answer.
The remaining letters 'keep' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for upkeep that I've seen before include "Peek up at the maintenance" , "Running costs" , "Cost of maintaining" , "Looking after" , "Cost of maintenance, say household" .)

I've seen this clue in The Sun.
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