The issue is the wdFindAsk. Try .Wrap = wdFindStop instead.
And clean it up a little to bring it all inside the With...End With - I don't know why Microsoft records this abnormality
Code:
Sub clean()
With Selection.Find
.ClearFormatting
.Replacement.ClearFormatting
.Text = "^p"
.Replacement.Text = " "
.Forward = True
.Wrap = wdFindStop
.Format = False
.MatchCase = False
.MatchWholeWord = False
.MatchKashida = False
.MatchDiacritics = False
.MatchAlefHamza = False
.MatchControl = False
.MatchWildcards = False
.MatchSoundsLike = False
.MatchAllWordForms = False
.Execute Replace:=wdReplaceAll
End With
End Sub