3 that is second class (7)
I believe the answer is:
species
'class' is the definition.
(thesaurus)
'3 that is second' is the wordplay.
'e' becomes 'spec' (I can't justify this - if you can you should believe this answer much more).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'second' becomes 's' (common abbreviation).
'spec'+'ie'+'s'='SPECIES'
Can you help me to learn more?
(Other definitions for species that I've seen before include "Biological group" , "Taxonomic group capable of interbreeding" , "Breed or genus" , "Category" , "Taxonomic group whose members can interbreed" .)