Improving quality of teacher during strange reign (11)

I believe the answer is:

remastering

I'm a little stuck... Click here to teach me more about this clue!

'improving quality' is the definition.
The definition suggests a present participle verb which matches the answer.

'teacher during strange reign' is the wordplay.
'teacher' becomes 'master' (I've seen this before).
'during' is an insertion indicator.
'strange' is an anagram indicator.
'reign' is an anagram of 'reing'.
'master' going within 'reing' is 'REMASTERING'.

'of' acts as a link.

Can you help me to learn more?

I've seen this clue in The Daily Express.

Want a hint initially instead of a full solution? Install my app