... family, ___, species (5)
I believe the answer is:
genus
I believe this is a double definition.
'family' is the first definition.
(thesaurus)
'species' is the second definition.
(thesaurus)
(Other definitions for genus that I've seen before include "Division of family" , "Latin class?" , "Taxonomic grouping" , "Taxonomic group (biol.)" , "group of organisms" .)