Enclosure with label on - a five-sided shape (8)
I believe the answer is:
pentagon
'a five-sided shape' is the definition.
(I've seen this before)
'enclosure with label on' is the wordplay.
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'with' means one lot of letters go next to another.
'label' becomes 'tag' (synonyms).
'pen'+'tag'+'on'='PENTAGON'
(Other definitions for pentagon that I've seen before include "US government building" , "Five-sided shape" , "military installation" , "rectangle snipped at one corner?" , "government HQ abroad" .)