Author at game with cricket side in form (8)
I believe the answer is:
pentagon
'form' is the definition.
(pentagon is a kind of form)
'author at game with cricket side' is the wordplay.
'author' becomes 'pen' (to pen something is to author it).
'at' says to put letters next to each other.
'game' becomes 'tag' (children's playground game).
'with' means one lot of letters go next to another.
'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 "military leadership" , "military building" , "US government building" , "Five-sided shape" , "Figure in plane" .)