Slander retracted at start - surprise (bigly) for Trump (8)

I believe the answer is:

dumfound

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'surprise bigly for trump' is the definition.
I can't judge whether this defines the answer.

'slander retracted at start' is the wordplay.
'slander' becomes 'mud' (as in to drag someone's name through the mud).
'retracted' says the letters should be written backwards.
'at' says to put letters next to each other.
'start' becomes 'found' (I've seen this before).
'mud' written backwards gives 'dum'.
'dum'+'found'='DUMFOUND'

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I've seen this clue in The Independent.

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