Add ... Frank unknown (5)
I believe the answer is:
annex
'add' is the definition.
(I've seen this in another clue)
'frank unknown' is the wordplay.
'frank' becomes 'Anne' (Anne Frank, Holocaust victim).
'unknown' becomes 'x' (algebra).
'anne'+'x'='ANNEX'
(Other definitions for annex that I've seen before include "Seize" , "Tack on" , "Add to one's territory" , "Take by force" , "Take (land) by conquest" .)