The most common and most likely answer to this crossword clue is the 8 letter word UNUSABLE
I've seen this crossword clue in The Guardian
U
N
U
S
A
B
L
E
Buddy explains!
The straight part
We know that "not fit for purpose" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not fit for purpose = unusable.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 UNUSABLE mean?
UNUSABLE adjective- Not usable.