Also in Pandora's Box (3)
I believe the answer is:
and
'also' is the definition.
(I've seen this before)
'in pandora's box' is the wordplay.
'in' indicates the answer is hidden within the clue.
'AND' is hidden within 'pandoras box'.
(Other definitions for and that I've seen before include "Together with" , "Also" , "Joining word" , "Plus" , "It's used to connect words, clauses or sentences" .)