The most common and most likely answer to this crossword clue is the 4 plus 2 plus 6 letter word SAFE AS HOUSES
I've seen this crossword clue in Financial Times
S
A
F
E
A
S
H
O
U
S
E
S
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 = safe as houses.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.