The most common and most likely answer to this crossword clue is the 8 letter word CASUALTY
I've seen this crossword clue in Financial Times
C
A
S
U
A
L
T
Y
More clues leading to the result CASUALTY
- Irregular therapy disheartened sufferer
- ‘Lost’, say — a cult TV series
- In California, flipping USA, temperature variable, in ER
- TV series causes part-time politician to lose heart
- Victim unconcerned when they take man away
- Informal essay overlooking core department of hospital
- Dreadfully slay with a cut, producing this?
- Careless type half-cut in hospital department
- Indifferent – those people lacking heart for victim
Buddy explains!

The straight part
We know that "in ER" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in er = casualty.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 CASUALTY mean?
CASUALTY noun- Something that happens by chance, especially an unfortunate event; an accident, a disaster.
- A person suffering from injuries or who has been killed due to an accident or through an act of violence.
- Specifically, a person who has been killed (not only injured) due to an accident or through an act of violence; a fatality.
- A person in military service who becomes unavailable for duty, for any reason (notably death, injury, illness, capture, or desertion).
- The accident and emergency department of a hospital.
- An incidental charge or payment.
- Chance nature; randomness.