The most common and most likely answer to this crossword clue is the 6 letter word CADDIS
I've seen this crossword clue in The Guardian
C
A
D
D
I
S
Buddy explains!

The straight part
We know that "immature creature" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case immature creature = caddis.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 CADDIS mean?
CADDIS noun- The larva of a caddis fly. They generally live in cylindrical cases, open at each end, and covered externally with debris.
- A rough woolen cloth; caddice.
- A kind of worsted lace or ribbon.