Cage with label on — it's five-sided (8)
I believe the answer is:
pentagon
'five-sided' is the definition.
I can't tell whether this definition defines the answer.
'cage with label on' is the wordplay.
'cage' becomes 'pen' (both can mean an enclosure).
'with' is a charade indicator (letters next to each other).
'label' becomes 'tag' (synonyms).
'pen'+'tag'+'on'='PENTAGON'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for pentagon that I've seen before include "Figure - US military HQ" , "US building of high security" , "US military leaders" , "Headquarters of the US Defence Department" , "Figure in plane" .)