In addition, it's contained in primordial soup (4)
I believe the answer is:
also
'in addition' is the definition.
(synonyms)
'contained in primordial soup' is the wordplay.
'contained in' indicates the answer is hidden within the clue.
'ALSO' is hidden within 'primordial soup'.
'it's' is the link.
(Other definitions for also that I've seen before include "As well, in addition" , "Item" , "revolutionary a part for Yugoslavia" , "Old" , "Furthermore" .)