The most common and most likely answer to this crossword clue is the 3 plus 6 letter word RED DEVILS
I've seen this crossword clue in The Guardian
R
E
D
D
E
V
I
L
S
More clues leading to the result RED DEVILS
Buddy explains!
The straight part
We know that "Display unit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case display unit = red devils.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.