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

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

'slander retracted at start' is the wordplay.
'slander' becomes 'mud' (as in to drag someone's name through the mud).
'retracted' shows that the letters should be reversed in order.
'at' means one lot of letters go next to another.
'start' becomes 'found' (I've seen this before).
'mud' back-to-front is 'dum'.
'dum'+'found'='DUMFOUND'

Can you help me to learn more?