In building, flagged excessive agent's fee (4,10)
I believe the answer is:
high commission
'in building flagged' is the definition.
(a High Commission works in an embassy with a flag)
'excessive agent's fee' is the wordplay.
'excessive' becomes 'high'.
'agent's fee' becomes 'commission'.
'high'+'commission'='HIGH COMMISSION'
(Other definitions for high commission that I've seen before include "What greedy salesman charges" , "Indian embassy in London, say" .)