A little optional something in addition (4)
I believe the answer is:
also
'in addition' is the definition.
(synonyms)
'a little optional something' is the wordplay.
'a little' indicates a hidden word.
'ALSO' is hidden in the letters of 'optional something'.
(Other definitions for also that I've seen before include "Additionally" , "Furthermore" , "an indication of more to come" , "in accompaniment" , "into the bargain" .)