Month in the past, found new shape (7)
I believe the answer is:
decagon
'shape' is the definition.
(10-sided shape)
'month in the past found new' is the wordplay.
'month' becomes 'dec' (abbreviation for December).
'in the past' becomes 'ago' (synonyms).
'found' is a charade indicator (letters next to each other).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'dec'+'ago'+'n'='DECAGON'
(Other definitions for decagon that I've seen before include "Ten-sided plane figure" , "Figure with 10 sides" , "Ten-sided shape" , "10-sided figure" , "A polygon -- eg can-do (anag)" .)