Describing multisided shape in lagoon, act strangely (9)
I believe the answer is:
octagonal
'describing multisided shape' is the definition.
'lagoon act strangely' is the wordplay.
'strangely' is an anagram indicator.
'lagoon'+'act'='lagoonact'
'lagoonact' with letters rearranged gives 'OCTAGONAL'.
'in' acts as a link.
(Other definitions for octagonal that I've seen before include "with certain sides" , "Having eight straight sides" , "Eight sided" , "with quite a few sides" , "With eight sides and angles" .)