Lament that it's at capacity by the end of the day, unhappily (10)

I believe the answer is:

mournfully

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'unhappily' is the definition.
Both the answer and definition are adverbs. Perhaps they are linked in a way I don't understand?

'lament that it's at capacity by the end of the day' is the wordplay.
'lament' becomes 'mourn' (I've seen this in another clue).
'that it's at capacity' becomes 'full' (I've seen this in another clue).
'by' means one lot of letters go next to another.
'the end of the' suggests the final letters.
The last letter of 'day' is 'y'.
'mourn'+'full'+'y'='MOURNFULLY'

Can you help me to learn more?

(Another definition for mournfully that I've seen is " Sadly".)

I've seen this clue in The Irish Times.

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