Sister goes back around one in the afternoon to get a light splitter (5)
I believe the answer is:
prism
'a light splitter' is the definition.
(I've seen this before)
'sister goes back around one in the afternoon' is the wordplay.
'back' suggests the final letters.
'around' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'the afternoon' becomes 'pm'.
The last letters of 'sister goes' are 'rs'.
'rs' going around 'i' is 'ris'.
'ris' put inside 'pm' is 'PRISM'.
'to get' acts as a link.
(Other definitions for prism that I've seen before include "Shape used to disperse light beams" , "What's on Dark Side of the Moon" , "Disperser or reflector of light" , "Light transmitter" , "Refracting glass" .)