The most common and most likely answer to this crossword clue is the 9 letter word QUADRUPED
I've seen this crossword clue in The Times
Q
U
A
D
R
U
P
E
D
Buddy explains!

The straight part
We know that "I’m four foot one" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i’m four foot one = quadruped.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does QUADRUPED mean?
QUADRUPED noun- A four-footed or four-legged animal
- A mammal ambulating on all fours