Broadcast travelling martians left intercepted by telegraphist following evacuation (11)

I believe the answer is:

transmittal

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'martians left intercepted by telegraphist following evacuation' is the definition.
I can't tell whether this definition defines the answer.

'broadcast travelling' is the wordplay.
'broadcast' becomes 'transmit' (synonyms).
'travelling' becomes 'tal' (I can't explain this - if you can you should believe this answer much more).
'transmit'+'tal'='TRANSMITTAL'

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