Figure initially crawling past into lair (7)
I believe the answer is:
Here is my best explanation:
'figure' is the definition.
(decagon is a kind of figure)
'initially crawling past into lair' is the subsidiary indication.
'initially' says to take the initial letters.
'past' becomes 'ago' ('ago' can be similar in meaning to 'past').
'into' is an insertion indicator.
'lair' becomes 'den' ('den' can be a synonym of 'lair').
The initial letter of 'crawling' is 'c'.
'cago' put into 'den' is 'decagon'.
(Other definitions for decagon that I've seen before include "Geometric figure with ten sides", "A polygon -- eg can-do (anag)", "Figure with 10 sides", "Go, dance on geometric figure with ten sides", "10-sided figure", "Plane shape", "Ten-sided plane figure" and "Ten-sided shape".)