The most common and most likely answer to this crossword clue is the 5 letter word EIDER
I've seen this crossword clue in The Times
E
I
D
E
R
More clues leading to the result EIDER
- Does it grow down as it grows up?
- Occasionally behind peers: feel down about this?
- Bird’s colourful — that is, on reflection
- Duck's colour that is reflected
- Sea duck behind heard at regular intervals
- It grows down river in Germany
- Swimmer's about to perish, heading west
- Visibly embarrassed, that is, after recalling duck
- Duck always found around one delta
- About to pass on, looking up and down supplier?
Buddy explains!

The straight part
We know that "Does it grow down as it grows up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case does it grow down as it grows up = eider.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 EIDER mean?
EIDER noun- Any of the species of the genera Polysticta or Somateria, in the seaduck subfamily Merginae, which line their nests with fine down (taken from their own bodies).