Part of immortal soul too? (4)
I believe the answer is:
also
'too?' is the definition.
(synonyms)
'part of immortal soul' is the wordplay.
'part of' says the answer is hidden in the clue.
'ALSO' is hidden in the letters of 'immortal soul'.
(Other definitions for also that I've seen before include "Old" , "To boot" , "'Besides, moreover (4)'" , "Item" , "an indication of more to come" .)