Unknown lives in Dutch region, victim of Viking raid (4,6)
I believe the answer is:
holy island
'victim of viking raid' is the definition.
The definition suggests a singular noun which matches the answer.
'unknown lives in dutch region' is the wordplay.
'unknown' becomes 'y' (algebra).
'lives' becomes 'is' ('be' can be a synonym of 'live').
'in' means one lot of letters goes inside another.
'dutch region' becomes 'holland' (Holland is an example. I am not sure about the 'dutch' bit.).
'y'+'is'='yis'
'yis' going inside 'holland' is 'HOLY ISLAND'.
Can you help me to learn more?
(Another definition for holy island that I've seen is " Lindisfarne".)