In addition, as well (3)
I believe the answer is:
too
I believe this clue is a double definition.
'in addition' is the first definition.
(I've seen this before)
'as well' is the second definition.
(both can mean 'also')
(Other definitions for too that I've seen before include "Additionally" , "As well, also" , "Excessively" , "What a pity!" , "As well; in addition" .)