Five-sided polygon (8)
I believe the answer is:
'five-sided polygon' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pentagon that I've seen before include "US building of high security" , "Location of the US Department of Defense" , "Five-sided figure - US Department of Defense" , "Figure - US military HQ" , "Figure in plane" .)