Instead of Cells(P+1.Row, "A") it's Cells(P.Row + 1, "A")
P.Row evaluates to a number upon which you can perform any arithmetic operation.
However, keep in mind that if you add that line, it's going to clear every Cells(P.Row + 1, "A") even if there isn't an NaN in Cells(P.Row + 1, "M").
In other words, will you always have extraneous data in Cells(P.Row + 1, "A") that you will want to clear?
P.S. If you plan to add that line, it's more efficient to check P once and perform whatever actions you want to perform based on that single test. In other words, code it like this:
'Find last Row in Column 13
lastRow = Range("M" & Rows.Count).End(xlUp).Row
For Each P In Range("M1:M" & lastRow)
'Delete Data In A if M = NaN
If P = "NaN" Then
Cells(P.Row + 1, "A").ClearContents