The most common and most likely answer to this crossword clue is the 8 letter word CONSTANT
I've seen this crossword clue in Financial Times
C
O
N
S
T
A
N
T
More clues leading to the result CONSTANT
- Loyal worker upholding Conservative way
- Dependable and firm, the moment I left
- Cold worker keeping on, good fellow — and steadfast
- Regular adviser university left out
- Dependable and firm the moment one leaves
- Studies article going into race being fixed
- Interminable nonsense about on street
- Uniform on, get to one’s feet briefly in court
- Carefully studies article going into race being fixed
- Dependable and firm, moment one departs
Buddy explains!

The straight part
We know that "Dependable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dependable = constant.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 CONSTANT mean?
CONSTANT noun- That which is permanent or invariable.
- A quantity that remains at a fixed value throughout a given discussion.
- Any property of an experiment, determined numerically, that does not change under given circumstances.
- An identifier that is bound to an invariant value; a fixed value given a name to aid in readability of source code.
- Unchanged through time or space; permanent.
- Consistently recurring over time; persistent.
- Steady in purpose, action, feeling, etc.
- Firm; solid; not fluid.
- Consistent; logical.
- (complexity theory) Bounded above by a constant.