A mirror's shattering interrupts dull morning letter opening (4,3,2,5)

I believe the answer is:

dear sir or madam

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

'letter opening' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'a mirror's shattering interrupts dull morning' is the wordplay.
'shattering' is an anagram indicator.
'interrupts' means one lot of letters goes inside another.
'dull' becomes 'dead' (I've seen this before).
'morning' becomes 'am' (as in de:00 AM).
'a'+'mirrors'='amirrors'
'amirrors' anagrammed gives 'rsirorma'.
'rsirorma' placed within 'dead' is 'dearsirormad'.
'dearsirormad'+'am'='DEAR SIR OR MADAM'

Can you help me to learn more?

(Another definition for dear sir or madam that I've seen is " Formal introduction".)

I've seen this clue in The Telegraph.

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