The most common and most likely answer to this crossword clue is the 9 letter word GHETTOISE
I've seen this crossword clue in The Guardian
G
H
E
T
T
O
I
S
E
Buddy explains!
The straight part
We know that "be very discriminating" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be very discriminating = ghettoise.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 GHETTOISE mean?
GHETTOISE verb- To put (someone) in a ghetto, or to isolate as if in a ghetto.
- To make (a place) into a ghetto, or to add the characteristics of a ghetto.