A fatal combination of words (5,8)

I believe the answer is:

death sentence

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'a fatal' is the definition.
I can't judge whether this definition defines the answer.

'combination of words' is the wordplay.
'combination of' becomes 'death' (I am not sure about this - if you are sure you should believe this answer much more).
'words' becomes 'sentence' (I've seen this before).
'death'+'sentence'='DEATH SENTENCE'

Can you help me to learn more?

(Another definition for death sentence that I've seen is " There may be another killing after this".)

I've seen this clue in The Irish Independent.

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