The most common and most likely answer to this crossword clue is the 2 plus 5 plus 2 plus 5 letter word AS GREEN AS GRASS
I've seen this crossword clue in The Guardian
A
S
G
R
E
E
N
A
S
G
R
A
S
S
Buddy explains!
The straight part
We know that "Very inexperienced" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very inexperienced = as green as grass.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.