The most common and most likely answer to this crossword clue is the 6 letter word MAMMAL
I've seen this crossword clue in The Times
M
A
M
M
A
L
More clues leading to the result MAMMAL
- Man is one left to support old lady
- Animal in line following mother
- Cricket isn’t one but bat is
- Mother childishly left animal
- Its mother’s first to lactate
- Mother left animal
- Animal has mother over 50
- Man, say, left supporting mother
- One in 3,000 - first-class liberal man, for example
- Two mothers start to lactate. This provides milk for the brood
Buddy explains!

The straight part
We know that "Man is one" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case man is one = mammal.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 MAMMAL mean?
MAMMAL noun- An animal of the class Mammalia, characterized by being warm-blooded, having hair and feeding milk to its young.
- A vertebrate with three bones in the inner ear and one in the jaw.