Sabre fitted with guard primarily that gleams (3,5)
I believe the answer is:
cut glass
'that gleams' is the definition.
(cut glass may gleam or glimmer)
'sabre fitted with guard primarily' is the wordplay.
'sabre' becomes 'cutlass' (both can mean a blade).
'fitted with' is an insertion indicator.
'primarily' indicates taking the first letters.
The first letter of 'guard' is 'g'.
'cutlass' enclosing 'g' is 'CUT GLASS'.
(Other definitions for cut glass that I've seen before include "Was crafted in Waterford" , "(Of accent) precisely enunciated" , "ornamental tableware" , "Posh" , "Vases or bowls decorated by grinding - (of accent) upper-class" .)