Mounting castle maintenance (6)

I believe the answer is:

upkeep

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'maintenance' is the definition.
(I know that maintenance can be written as upkeep)

'mounting castle' is the wordplay.
'mounting' becomes 'up' (I can't explain this - if you can you should believe this answer much more).
'castle' becomes 'keep' (keep is a type of castle).
'up'+'keep'='UPKEEP'

Can you help me to learn more?

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

I've seen this clue in The Telegraph.

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