The most common and most likely answer to this crossword clue is the 6 letter word EMBERS
I've seen this crossword clue in The Daily Telegraph
E
M
B
E
R
S
Buddy explains!

The straight part
We know that "Remains" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case remains = embers.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 EMBERS mean?
EMBERS noun- A glowing piece of coal or wood.
- Smoldering ash.
- The smoldering or glowing remains of a fire, smoldering ash.