The most common and most likely answer to this crossword clue is the 15 letter word COPPER-BOTTOMED
I've seen this crossword clue in Financial Times
C
O
P
P
E
R
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B
O
T
T
O
M
E
D
More clues leading to the result COPPER-BOTTOMED
Buddy explains!
The straight part
We know that "Reliable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reliable = copper-bottomed.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 COPPER-BOTTOMED mean?
COPPER-BOTTOMED adjective- Having lower parts made of or covered by copper (especially of a ship or cookware).
- Thoroughly reliable; secure