The most common and most likely answer to this crossword clue is the 7 letter word PIARIST
P
I
A
R
I
S
T
Buddy explains!
The straight part
We know that "One instructing poor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one instructing poor = piarist.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.