Plural is in shares (3)
I believe the answer is:
are
'plural is' is the definition.
(plural form of the verb 'is')
'in shares' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ARE' can be found hidden inside 'shares'.
(Other definitions for are that I've seen before include "is for more than one" , "(We/you/they) exist" , "Exist; square measure" , "Unit of land measure equal to 100 square metres" , "live" .)