Class that is wearing glasses (7)
I believe the answer is:
species
'class' is the definition.
(thesaurus)
'that is wearing glasses' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'wearing' means one lot of letters goes inside another.
'glasses' becomes 'specs' ('specs' can be a synonym of 'glasses').
'ie' placed into 'specs' is 'SPECIES'.
(Other definitions for species that I've seen before include "Category" , "Taxonomic group capable of interbreeding" , "Kind of creature" , "Variety, taxonomic group" , "Kinds or sorts" .)