The most common and most likely answer to this crossword clue is the 8 letter word TRIMETER
I've seen this crossword clue in The Guardian
T
R
I
M
E
T
E
R
More clues leading to the result TRIMETER
Buddy explains!

The straight part
We know that "three feet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case three feet = trimeter.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 TRIMETER mean?
TRIMETER noun- A line in a poem having three metrical feet.
- A poetic metre in which each line has three feet.