Writer attending game: cricket side in shape (8)
I believe the answer is:
pentagon
'shape' is the definition.
(I know that pentagon is a type of polygon)
'writer attending game cricket side' is the wordplay.
'writer' becomes 'pen' (something which writes).
'attending' means one lot of letters go next to another.
'game' becomes 'tag' (children's playground game).
'cricket side' becomes 'on' (cricket term, on or leg side).
'pen'+'tag'+'on'='PENTAGON'
'in' acts as a link.
(Other definitions for pentagon that I've seen before include "rectangle snipped at one corner?" , "Headquarters of the US Defence Department" , "US DoD HQ" , "government HQ abroad" , "Figure in plane" .)