Pater's double start (4)

I believe the answer is:

papa

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'pater's' is the definition.
(synonyms)

'double start' is the wordplay.
'double' shows that letters should be duplicated.
'start' becomes 'pa' (I can't explain this - if you can you should give a lot more credence to this answer).
'pa' duplicated is 'PAPA'.

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(Other definitions for papa that I've seen before include "Father; letter P" , "Dad" , "Pop" , "Comms code for 'P'" , "one after Oscar" .)

I've seen this clue in the Evening Standard.

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