For all assembled round the compound, proves fatal (6)

I believe the answer is:

lethal

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'fatal' is the definition.
(similar in meaning)

'for all assembled round' is the wordplay.
'for' becomes 'eth' (I can't justify this - if you can you should believe this answer much more).
'assembled' indicates anagramming the letters.
'round' means one lot of letters goes inside another.
'all' anagrammed gives 'lal'.
'eth' inserted into 'lal' is 'LETHAL'.

'the compound proves' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for lethal that I've seen before include "Terminal" , "describing a killer?" , "Deadly or mortal" , "Noxious" , "Deadly dangerous" .)

I've seen this clue in the Evening Standard.

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