In addition, too (4)
I believe the answer is:
also
This clue is a double definition.
'in addition' is the first definition.
(synonyms)
'too' is the second definition.
(synonyms)
(Other definitions for also that I've seen before include "Additionally" , "As well, in addition" , "'Besides, moreover (4)'" , "Item" , "admitted to Royal Society > too" .)