An additional; one more (7)
I believe the answer is:
another
I believe this clue is a double definition.
'an additional' is the first definition.
(thesaurus)
'one more' is the second definition.
(I've seen this before)
(Other definitions for another that I've seen before include "A different one" , "Additional: alternative" , "Additional, one more" , "Somebody else" , "No heart (anag.)" .)