Incomplete common groups of species (6)
I believe the answer is:
genera
'groups of species' is the definition.
(I've seen this in another clue)
'incomplete common' is the wordplay.
'incomplete' means to remove the last letter.
'common' becomes 'general' (I've seen this before).
'general' with its final letter taken away is 'GENERA'.
(Other definitions for genera that I've seen before include "Taxonomic categories (biology)" , "Groups with common characteristics" , "Biological groupings" , "Sorts" , "Categories making up a family" .)