The most common and most likely answer to this crossword clue is the 4 plus 2 plus 5 plus 5 letter word LOVE AT FIRST SIGHT
I've seen this crossword clue in Financial Times
L
O
V
E
A
T
F
I
R
S
T
S
I
G
H
T
More clues leading to the result LOVE AT FIRST SIGHT
Buddy explains!

The straight part
We know that "Instant good feeling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instant good feeling = love at first sight.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.