Tosses in (4)
I believe the answer is:
adds
'tosses in' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for adds that I've seen before include "Contributes further" , "Finds the sum (4)," , "Aggregates or appends" , "Puts (numbers) together" , "Calculates the sum" .)