A month back, new plane shape (7)
I believe the answer is:
octagon
'plane shape' is the definition.
'octagon' can be an answer for 'shape' (octagon is a kind of polygon). I'm unsure of the 'plane' bit.
'a month back new' is the wordplay.
'a month' becomes 'oct' (abbreviation for October).
'back' becomes 'ago' ('years back' means 'years ago').
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'oct'+'ago'+'n'='OCTAGON'
Can you help me to learn more?
(Other definitions for octagon that I've seen before include "Eight sided figure" , "plane figure" , "Eight-sided room" , "Figure with 8 sides" , "Shape of a square with the corners cut off" .)