how to show only the first letter of an element of a string

I have a list of words i.e. [‘cat’, ‘dog’, ‘chicken’] and I want to replace all letters except the first with an underscore.
I tried using for loops but I couldn’t get it to work.

>Solution :

x = ['cat', 'dog', 'chicken']

y = [word[0] + '_'*(len(word)-1) for word in x]

print(y)
# ['c__', 'd__', 'c______']