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