The most common and most likely answer to this crossword clue is the 8 letter word IN-BUILT
I
N
-
B
U
I
L
T
Buddy explains!
The straight part
We know that "Fixed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fixed = in-built.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 IN-BUILT mean?
IN-BUILT adjective- Existing as an essential constituent; built-in; inherent; integral.