Fold to add shape (8)
I believe the answer is:
pentagon
'shape' is the definition.
(pentagon is a kind of shape)
'fold to add' is the wordplay.
'fold' becomes 'pen' (both can mean an animal enclosure).
'to add' becomes 'tagon' (to tag something on is to add it).
'pen'+'tagon'='PENTAGON'
(Other definitions for pentagon that I've seen before include "Five-sided shape" , "military building" , "Location of the US Department of Defense" , "US building of high security" , "Five-sided polygon" .)