The most common and most likely answer to this crossword clue is the 7 letter word LOCATOR
I've seen this crossword clue in The Times
L
O
C
A
T
O
R
Buddy explains!
The straight part
We know that "part of index" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of index = locator.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 LOCATOR mean?
LOCATOR noun- One who, or that which, locates.
- One who locates, or is entitled to locate, land or a mining claim.
- (travel industry) The unique alphanumeric reference given to each travel booking.