is the lake known as the "Mirrormere", found in the Dimrill Dale on the eastern side outside Khazad-dûm.  It was these waters in which Durin saw the crown of stars above his reflection, even though it was daytime.  From that sign, he took that he should found his kingdom at this site.  Tolkien translates Kheled-zâram as "probably 'glass-pool, lake' ".  This lines up with the name "Mirrormere".

In his notes, Tolkien says "kheled was certain a D. word for 'glass', and seems to be the origin of S heleð 'glass' ".  Here, kheled would be composition form as the first element of the compound, and almost certainly nominative.  Its hard to say what the number would be.  The only other place we see the template CeCeC is in felek, which is a verb.  The translation "glass" could be a singular or collective number in form, or perhaps it could be a separate form for mass or uncountable nouns such as materials. 
Arabic has nouns that are considered "mass nouns", and these have singular and plural forms.  From this, I would say that kheled is probably a singular form but can be used to indicate an uncounted amount of glass.

Kheled then should be "glass" and singular, nominaive, composition.

Zâram is translated as "lake, pool" without further comment.  As the second element of the compound, it is probably singular, nominative, indefinite.

Kheled-zâram is "glass-pool, lake" and a compound word with noun-noun word order.

The Return of the Shadow, pg 466
Parma Eldalamberon XVII: Words, Phrases and Passages, pg 35, 37
A Tolkien Compass, pg 190