Write label attached to figure (8)
I believe the answer is:
pentagon
'figure' is the definition.
(pentagon is a kind of figure)
'write label attached' is the wordplay.
'write' becomes 'pen' ('pen' can be a synonym of 'write').
'label' becomes 'tag' ('tag' can be a synonym of 'label').
'attached' becomes 'on' (I've seen this in other clues).
'pen'+'tag'+'on'='PENTAGON'
'to' is the link.
(Other definitions for pentagon that I've seen before include "US building of high security" , "military installation" , "military leadership" , "Headquarters of the US Defence Department" , "rectilineal figure" .)