Shape of writing implement with label attached (8)
I believe the answer is:
pentagon
'shape' is the definition.
(pentagon is a kind of shape)
'writing implement with label attached' is the wordplay.
'writing implement' becomes 'pen' (pen is a kind of writing implement).
'with' means one lot of letters go next to another.
'label' becomes 'tag' ('tag' can be a synonym of 'label').
'attached' becomes 'on' (I've seen this in other clues).
'pen'+'tag'+'on'='PENTAGON'
'of' acts as a link.
(Other definitions for pentagon that I've seen before include "US military leaders" , "Figure - US military HQ" , "government HQ abroad" , "US government building" , "Headquarters of the US Defense Department" .)