Part of Royal Society in addition (4)
I believe the answer is:
also
'in addition' is the definition.
(synonyms)
'part of royal society' is the wordplay.
'part of' indicates a hidden word.
'ALSO' can be found hidden inside 'royal society'.
(Other definitions for also that I've seen before include "Old" , "What's more" , "Plus" , "in accompaniment" , "revolutionary a part for Yugoslavia" .)