Only for education purpose, it's possible to create a seed with only one word.
Example:
action action action action action action action action action action action action
And have a lot of similar cases.
Using the word action and the site iancoleman, the time to crack is only 65 years, in this case not so secure...
If you manually create a mnemonic like this, then it is no longer a "seed" but a brainwallet and it is not safe at all because there is no randomness in the entropy that was used at all.
The time it takes to crack it is also in seconds not years because all it takes is someone trying all the combinations of 2048 words where all words are the same (check 2048 seed phrases in total).