The most common and most likely answer to this crossword clue is the 5 plus 4 letter word UNION LIST
I've seen this crossword clue in The Guardian
U
N
I
O
N
L
I
S
T
Buddy explains!
The straight part
We know that "subject inventory" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case subject inventory = union list.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.