Deadly apartment was unsuccessful (4,4)

I believe the answer is:

fell flat

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'unsuccessful' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives can occasionally define each other.

'deadly apartment' is the wordplay.
'deadly' becomes 'fell' (I've seen this before).
'apartment' becomes 'flat' (synonyms).
'fell'+'flat'='FELL FLAT'

'was' acts as a link.

Can you help me to learn more?

(Other definitions for fell flat that I've seen before include "failed to impress" , "provoked no interest" , "collapsed" .)

I've seen this clue in The Daily Express.

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