Skull's passing a note in the outhouse (6-4)

I believe the answer is:

deaths-head

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'skull's' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'passing a note in the outhouse' is the wordplay.
'passing' becomes 'death' (I've seen this in another clue).
'a note' becomes 'a' (musical note).
'in the' is an insertion indicator.
'outhouse' becomes 'shed' (a shed is an outside building).
'a' put inside 'shed' is 'shead'.
'death'+'shead'='DEATHS-HEAD'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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