Alternative, additional (5)
I believe the answer is:
other
I believe this is a double definition.
'alternative' is the first definition.
(I've seen this before)
'additional' is the second definition.
(I've seen this before)
(Other definitions for other that I've seen before include "Different, disparate" , "Alternative - unrelated" , "Distinct" , "Alternative of two" , "Not the one mentioned" .)