The most common and most likely answer to this crossword clue is the 5 plus 5 letter word PILOT LIGHT
I've seen this crossword clue in Financial Times
P
I
L
O
T
L
I
G
H
T
More clues leading to the result PILOT LIGHT
- Flame puts one group in dangerous situation
- Small flame: airman needs to land
- I’ll be in the garden, idle – but it shows one’s connected
- Irrational group lacking what should always be on
- It shows when working guide’s frivolous
- Flier of little importance gets auxiliary burner
- One group has difficulty concealing evidence of operation
Buddy explains!
The straight part
We know that "what should always be on" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what should always be on = pilot light.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.