The most common and most likely answer to this crossword clue is the 8 letter word EMITTERS
I've seen this crossword clue in The Guardian
E
M
I
T
T
E
R
S
More clues leading to the result EMITTERS
Buddy explains!
The straight part
We know that "They send out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they send out = emitters.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 EMITTERS mean?
EMITTERS noun- That which emits something.
- One terminal of a bipolar transistor (BJT).