Substitutes (for) (4,2)
(Another definition for sits in that I've seen is " Takes part in demonstration".)
'substitutes for' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for replaces that I've seen before include "Puts back" , "Substtitutes, supplants" , "Substitutes, supplants" , "Substitutes one for another" .)