"Let's shake on it!” (4)
'let's shake on it' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for deal that I've seen before include "Bargain, contract" , "What business folk do" , "understanding" , "Pass around (e.g. cards)" , "Kent town" .)
(Other definitions for agreed that I've seen before include "Accorded" , "Corresponded" , "Concurred - tallied" , "Acceded" , "Assented, acquiesced" .)