The most common and most likely answer to this crossword clue is the 17 letter word SCRATCH-AND-SNIFF
I've seen this crossword clue in The Guardian
S
C
R
A
T
C
H
-
A
N
D
-
S
N
I
F
F
Buddy explains!
The straight part
We know that "as scent tested" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as scent tested = scratch-and-sniff.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.