Concealed in both ideas (3)
I believe the answer is:
hid
'concealed' is the definition.
('hide' can be a synonym of 'conceal')
'in both ideas' is the wordplay.
'in' indicates a hidden word.
'HID' can be found hidden inside 'both ideas'.
(Other definitions for hid that I've seen before include "Concealed, kept secret" , "Shrouded" , "Concealed from sight" , "Kept out of sight" , "Concealed (oneself?)" .)