At some stage, a fancy piece in the office (6)
I believe the answer is:
agency
'office' is the definition.
('agency' can be a synonym of 'office')
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for agency that I've seen before include "Office or business of one who does business for others" , "Government unit - business providing services" , "Business that conducts business on behalf of others" , "Business organisation" , "Intervention producing a particular effect" .)