Old girl packing beginning to idly stuff in rubies, sapphires etc. (7)
I believe the answer is:
alumina
'stuff in rubies sapphires etc' is the definition.
I can't tell whether this definition defines the answer.
'old girl packing beginning to idly' is the wordplay.
'old girl' becomes 'alumna' (I've seen this before).
'packing' means one lot of letters goes inside another (some letters are packed inside).
'beginning to' indicates taking the first letters.
The initial letter of 'idly' is 'i'.
'alumna' placed around 'i' is 'ALUMINA'.
Can you help me to learn more?
(Other definitions for alumina that I've seen before include "Substance of sapphire" , "Metal oxide found in many clays" , "Oxide in corundum" , "Mineral in many clays" , "Abrasive" .)