The most common and most likely answer to this crossword clue is the 9 letter word SEEDLINGS
I've seen this crossword clue in The Daily Telegraph
S
E
E
D
L
I
N
G
S
Buddy explains!

The straight part
We know that "The littlest ones growing up in a nursery" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the littlest ones growing up in a nursery = seedlings.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 SEEDLINGS mean?
SEEDLINGS noun- A young plant grown from seed.
- Any young plant, especially: