Write name on official building (8)

I believe the answer is:

pentagon

I'm a little stuck... Click here to teach me more about this clue!

'official building' is the definition.
'pentagon' can be an answer for 'building' (Pentagon is an example). I am not certain of the 'official' bit.

'write name on' is the wordplay.
'write' becomes 'pen' (synonyms).
'name' becomes 'tag' (I've seen this in other clues).
'pen'+'tag'+'on'='PENTAGON'

Can you help me to learn more?

(Other definitions for pentagon that I've seen before include "Geometric figure with five sides" , "US DoD HQ" , "top brass" , "military building" , "Headquarters of the US Defence Department" .)

I've seen this clue in The Guardian.

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