how cut strings after a specific word?

You can use str.split to split on see less and then use str slicing to take the first part.

df['col'] = df['col'].str.split('see less').str[0]

Output:

                 col
0          hi there 
1  today is summer, 

You can use a simple regex and str.replace:

df['col'] = df['col'].str.replace('see less.*', '', regex=True)

output:

                 col
0          hi there 
1  today is summer,

If you also want to remove the non-letter character just before:

df['col'].str.replace(r'\W+see less.*', '', regex=True)

output:

               col
0         hi there
1  today is summer

Use str.extract:

df['col'] = df['col'].str.extract('(.*)\s*see less')
print(df)

# Output
                 col
0          hi there 
1  today is summer,