Class of agents protecting European Commission (7)
I believe the answer is:
species
'class' is the definition.
(thesaurus)
'agents protecting european commission' is the wordplay.
'agents' becomes 'spies' (spy is a kind of agent).
'protecting' is an insertion indicator.
'european commission' becomes 'ec'.
'spies' placed around 'ec' is 'SPECIES'.
'of' is the link.
(Other definitions for species that I've seen before include "Taxonomic grouping" , "Sort, type" , "Kind of creature" , "Category" , "Kind, sort" .)