How do you replace all the occurrences of a certain character in a string?

I am reading a csv into a:

import csv
import collections
import pdb
import math
import urllib

def do_work():
  a=get_file('c:/pythonwork/cds/cds.csv')
  a=remove_chars(a)
  print a[0:10]

def get_file(start_file): #opens original file, reads it to array
  with open(start_file,'rb') as f:
    data=list(csv.reader(f))
  return (data)

def remove_chars(a):
  badchars=['a','b','c','d']
  for row in a:
    for letter in badchars:
      row[8].replace(letter,'')
  return a

I would like to replace all occurrences of ['a','b','c','d'] in the 8th element of the line with empty string. the remove_chars function is not working.

Is there a better way to do this?