"What's one plus one?" read out in addition (3)
I believe the answer is:
too
This clue might be a double definition.
'what's one plus' is the first definition.
(I've seen this in another clue)
'one? read out in addition' is the second definition.
'too' can be an answer for 'addition' (thesaurus). I'm not sure about the 'one? read out in' bit.
Can you help me to learn more?
(Other definitions for too that I've seen before include "As well, also" , "Moreover, in addition" , "On top - also" , "Excessively" , "Furthermore" .)