The most common and most likely answer to this crossword clue is the 8 letter word TRUSTFUL
I've seen this crossword clue in The Times
T
R
U
S
T
F
U
L
Buddy explains!

The straight part
We know that "gullible" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gullible = trustful.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 TRUSTFUL mean?
TRUSTFUL adjective- Trusting; willing to trust.
- Trustworthy.