Crossword Genius

Service charges for castle 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 castle in the air?' is the wordplay.
I cannot quite understand how this works, but
'castle' could be 'keep' (I've seen this in other clues) and 'keep' is located in the answer.
'in the air?' could be 'up' and 'up' is present in the answer.
No letters remain.
This explanation may well be incorrect...

Can you help me to learn more?

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

I've seen this clue in The Telegraph and The Sun.
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