Express Skeleton

[Buick88]

Here we go! Top right corner!
12a) ????...a ring with a cut. I have OPAL...(why?)
7d) G???P...Party mature women departed...I have GROUP...(why)
8d) N???A..New variable like an animal...I have NYALA...(why)
Bottom Left.
18d) A?O?T...Graduate turned away roughly...I have ABOUT...(why)
Thanks for your patience xxx

[Toorak]

7 group gro(w) up

[donny48]

18 ( BA graduate turned)ab+ out (away)

[Toorak]

About - ba ( graduate) reversed & out (away)

[ubasses]

O (ring) pal(e)

[dannyk13]

12a.Opal (don't think you have posted full clue) O + pal(e)
7d,Gro(wn)up
8d,N9new) +y (variable) + a la (like)
18d.Ba(graduate) reversed + out(away)

[cashier]

7 mature grown up women (wn) departed group

[Toorak]

N(ew) y ( variable) a la (like)

[ubasses]

N (ew) y (variable) ala (like)

[donny48]

8 (new) n + y(variable) + a la ( like )

[emmie]

The lowland nyala or simply nyala (Tragelaphus angasii),[3] is a spiral-horned antelope
not sure that gives the explanation.

[Buick88]

Thank you all so much. xxx