Fortified building with increased running costs (6)
I believe the answer is:
upkeep
'running costs' is the definition.
(maintenance costs)
'fortified building with increased' is the wordplay.
'fortified building' becomes 'keep' (keep can mean a fort or castle).
'with' says to put letters next to each other.
'increased' becomes 'up' (something that's up has increased or risen).
'keep' after 'up' is 'UPKEEP'.
(Other definitions for upkeep that I've seen before include "Cost of maintenance, say household" , "Peek up at the maintenance" , "Cost of maintaining" , "Running costs" , "Looking after" .)