I believe this is what you are looking for below, this will take a value from cell B1 and compare against values in Column A, and if the values are less than the value in B1, it will clear the contents of that cell:
Sub ClearLowerThan()
Dim c As Range, Rng As Range
Dim LastRow As Long
Dim ws As Worksheet: Set ws = Sheets("Sheet1")
'declare you worksheet, amend as required
LastRow = ws.Cells(ws.Rows.Count, "A").End(xlUp).Row
'get the last row with data on Column A
CompareVal = ws.Range("B1").Value
'get the value to compare against, in this case from cell B1
Set Rng = ws.Range("A1:A" & LastRow)
'set the range to compare against the value from B1
For Each c In Rng 'for each cell in the given range
If c.Value < CompareVal Then c.ClearContents
'if value of cell is less than the value to compare against, clear the cell contents.
Next
End Sub