Can't go wrong following old shape (7)
I believe the answer is:
octagon
'shape' is the definition.
(octagon is a kind of shape)
'can't go wrong following old' is the wordplay.
'wrong' indicates anagramming the letters.
'following' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cant'+'go'='cantgo'
'cantgo' with letters rearranged gives 'ctagon'.
'ctagon' after 'o' is 'OCTAGON'.
(Other definitions for octagon that I've seen before include "plane figure" , "Eight sided figure" , "Shape of a square with the corners cut off" , "Eight-sided shape" , "Eight-sided object" .)