I must abandon assumed name, sadly (4)

I believe the answer is:

alas

'sadly' is the definition.
('alas' is used to express regret)

'i must abandon assumed name' is the wordplay.
'must abandon' suggests deleting specific letters.
'assumed name' becomes 'alias' (synonyms).
'alias' with 'i' taken out is 'ALAS'.

(Other definitions for alas that I've seen before include "Oh dear!" , "Lackaday" , "Antipodean island state" , "Proper, respectable" , "'- Poor Yorick' (Hamlet)" .)

I've seen this clue in The Times.

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