Rising section of fortified site needs regular repairs (6)
I believe the answer is:
upkeep
'regular repairs' is the definition.
(upkeep means the cost maintenance or repairs)
'rising section of fortified site' is the wordplay.
'rising' becomes 'up' (similar in meaning).
'section of fortified site' becomes 'keep' (part of a castle).
'up'+'keep'='UPKEEP'
'needs' acts as a link.
(Other definitions for upkeep that I've seen before include "Running costs" , "Maintenance costs" , "Cost of support" , "Looking after" , "Peek up at the maintenance" .)