Maintenance of high castle (6)
I believe the answer is:
upkeep
'maintenance' is the definition.
(I know that maintenance can be written as upkeep)
'high castle' is the wordplay.
'high' becomes 'up' (associated in meaning).
'castle' becomes 'keep' (I've seen this in other clues).
'up'+'keep'='UPKEEP'
'of' is the link.
(Other definitions for upkeep that I've seen before include "Maintenance costs" , "payments?" , "Running costs" , "Looking after" , "Cost of maintaining" .)