Prime Minister stopped by retired teacher in shape (5)
I believe the answer is:
prism
'shape' is the definition.
(prism is a kind of shape)
'prime minister stopped by retired teacher' is the wordplay.
'prime minister' becomes 'pm' (synonyms).
'stopped by' indicates putting letters inside (I've seen this in other clues).
'retired' says the letters should be written backwards.
'teacher' becomes 'sir' (way of addressing a male teacher).
'sir' backwards is 'ris'.
'pm' going around 'ris' is 'PRISM'.
'in' acts as a link.
(Other definitions for prism that I've seen before include "Transparent 3 sided object that seperates light into colours" , "Crystal form" , "Light-splitting glass" , "Disperser or reflector of light" , "Cut-glass object" .)