Group that is breaking glasses (7)
I believe the answer is:
species
'group' is the definition.
(species is a kind of group)
'that is breaking glasses' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'breaking' indicates putting letters inside.
'glasses' becomes 'specs' ('specs' can be a synonym of 'glasses').
'ie' going into 'specs' is 'SPECIES'.
(Other definitions for species that I've seen before include "Sort, type" , "Biological grouping" , "Groups of related plants or animals" , "Living group; kind" , "Category" .)