Hi Hans,
processors aren’t applied to the second argument of \seeindex, only the first is formatted.
\defineprocessor[smallcaps][style=\sc]
\starttext
Apples\index{smallcaps->Apple}
Malus domestica\seeindex{smallcaps->Malus domestica}{smallcaps->Apple}
\placeindex[n=1]
\stoptext
Wolfgang