Month since North American League formed with eight sides (9)
I believe the answer is:
octagonal
'with eight sides' is the definition.
(an octagon has eight sides)
'month since north american league' is the wordplay.
'month' becomes 'oct' (abbreviation for October).
'since' becomes 'ago' (I've seen this in another clue).
'north american' becomes 'na'.
'league' becomes 'l' (abbreviation).
'oct'+'ago'+'na'+'l'='OCTAGONAL'
'formed' is the link.
(Other definitions for octagonal that I've seen before include "In unusual shape" , "With eight sides and angles" , "Eight-sided" , "with quite a few sides" , "Having eight sides as in geometric figure" .)