Caesar Cypher--- I don't understand why it doesn't work

[ironsheep]

alphabet= "abcdefghijklmnopqrstuvwxyz"
cleartext= input()
cleartext=cleartext_lowercase()
def encrypt(cleartext):
    codedtext = " "
    for character in cleartext:
            if character in alphabet:
                newcharacter= (alphabet.find(character) + 13) %26
                codedtext += alphabet[newcharacter]
            else:
                codedtext += character
            return codedtext
print(encrypt(cleartext))

I replaced the elif: with else and now my new error message is

Error:
Traceback (most recent call last):
  File "C:\Users\Mitesh Patel\Desktop\secret code.py", line 3, in <module>
    cleartext=cleartext_lowercase()
NameError: name 'cleartext_lowercase' is not defined