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Yes, here is a Python code that verifies if the string preceding the nth string in a provided list of strings is a correct substring:

def verify_substring(lst, n):
    if n == 0 or n >= len(lst):
        return False # if n is invalid, return False

    substring = lst[n-1] # get the nth string
    prefix = lst[n-2] # get the string before the nth string

    return substring.startswith(prefix) # check if prefix is a valid substring of substring

To use this code, simply call the verify_substring function with a list of strings and the index of the string you want to verify. For example:

strings = ["apple", "ape", "banana", "berry"]
index = 2
valid_prefix = verify_substring(strings, index) # should return True, since "banana" starts with "ape"

if valid_prefix:
    print("The prefix is a valid substring!")
else:
    print("The prefix is not a valid substring!")

Note that the function returns False if the index is invalid (i.e. less than 1 or greater than or equal to the length of the list).