The most common and most likely answer to this crossword clue is the 6 letter word TRUSTY
I've seen this crossword clue in The Guardian
T
R
U
S
T
Y
More clues leading to the result TRUSTY
- Reliable, but ultimately out of practice
- Model prisoner first of those getting out of nick
- Well-behaved prisoner and copper on time
- T-tarnished but reliable
- Reliable if time-worn?
- Dependable type of shirt suffering from lack of use
- Convict getting privileges can be relied on
- Bid to suppress beginnings of unresolved sexual tension, being faithful
- Devoted time, in need of practice
- Upright corroded by time
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 = trusty.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 TRUSTY mean?
TRUSTY noun- A trusted person, especially a prisoner who has been granted special privileges.
- Reliable or trustworthy.