Write graffiti artist’s signature on figure (8)
I believe the answer is:
pentagon
'figure' is the definition.
(type of geometrical figure)
'write graffiti artist's signature on' is the wordplay.
'write' becomes 'pen' (synonyms).
'graffiti artist's signature' becomes 'tag' (I've seen this in another clue).
'pen'+'tag'+'on'='PENTAGON'
(Other definitions for pentagon that I've seen before include "Figure - US military HQ" , "military building" , "Five-sided polygon" , "Location of the US Department of Defense" , "US DoD HQ" .)