Put two and two together, say (3)
This clue is a double definition.
'put two and two together' is the first definition.
(I've seen this before)
'say' is the second definition.
(adding is a kind of saying)
(Other definitions for add that I've seen before include "Say as a further remark" , "put two and two together" , "Sum, put together" , "Sum (numbers)" , "Tot; append" .)
(Other definitions for added that I've seen before include "Extra" , "Said" , "Summed up" , "Appended; totalled" , "Contributed" .)