The most common and most likely answer to this crossword clue is the 11 letter word TEAR-JERKER
I've seen this crossword clue in Independent
T
E
A
R
-
J
E
R
K
E
R
More clues leading to the result TEAR-JERKER
Buddy explains!
The straight part
We know that "source of emotional reaction" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case source of emotional reaction = tear-jerker.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 TEAR-JERKER mean?
TEAR-JERKER noun- An emotionally charged film, novel, song, opera, television episode, etc., usually with one or more sad passages or ending, so termed because it suggests one is likely to cry during its performance.