Writer on about label - a five-sided shape (8)
I believe the answer is:
pentagon
'a five-sided shape' is the definition.
(I've seen this before)
'writer on about label' is the wordplay.
'writer' becomes 'pen' (something which writes).
'about' is a charade indicator (letters next to each other).
'label' becomes 'tag' (tag is a kind of label).
'on' after 'tag' is 'tagon'.
'pen'+'tagon'='PENTAGON'
(Other definitions for pentagon that I've seen before include "rectangle snipped at one corner?" , "Five-sided shape" , "government HQ abroad" , "military building" , "A shape; Arlington HQ" .)