Such a message in album is unexpectedly left (10)

I believe the answer is:

subliminal

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'such a message' is the definition.
I can't tell whether this defines the answer.

'in album is unexpectedly left' is the wordplay.
'unexpectedly' is an anagram indicator.
'left' becomes 'l' (common abbreviation).
'in'+'album'+'is'='inalbumis'
'inalbumis' is an anagram of 'sublimina'.
'sublimina'+'l'='SUBLIMINAL'

Can you help me to learn more?

(Other definitions for subliminal that I've seen before include "sort of advertisement" , "Below the conscious level" .)

I've seen this clue in The Independent.

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