Numbers of species left common (7)
I believe the answer is:
general
'common' is the definition.
(I know that common can be written as general)
'numbers of species left' is the wordplay.
'numbers of species' becomes 'genera' (more than one biological genus).
'left' becomes 'l' (common abbreviation).
'genera'+'l'='GENERAL'
(Other definitions for general that I've seen before include "Not partial, local or particular like army chief" , "Not particular or specific" , "High-ranking officer" , "Usual; officer" , "Applying to all or most of a group" .)