Surprising French article about warning received by new witness (10)

'surprising' is the definition.
(thesaurus)

'french article about warning received by new witness' is the wordplay.
'french article' becomes 'un' (French for 'a' - the indefinite article).
'about' is an insertion indicator.
'warning' becomes 'fore' (I've seen this in other clues).
'received by' indicates putting letters inside.
'new witness' becomes 'nsee' (I can't justify this - if you can you should give a lot more credence to this answer).
'fore' put inside 'nsee' is 'nforesee'.
'un' going around 'nforesee' is 'UNFORESEEN'.

Can you help me to learn more?