Optical device returned, sir, during the afternoon (5)
I believe the answer is:
prism
'optical device' is the definition.
(I know that prism is a type of optical device)
'returned sir during the afternoon' is the wordplay.
'returned' shows that the letters should be reversed in order.
'during' indicates putting letters inside.
'the afternoon' becomes 'pm'.
'sir' reversed gives 'ris'.
'ris' put inside 'pm' is 'PRISM'.
(Other definitions for prism that I've seen before include "Light-bending glass" , "Cut-glass object" , "Light-splitting object" , "Triangular piece of glass, optical device" , "Triangular glass shape, breaks up light" .)