The most common and most likely answer to this crossword clue is the 6 plus 6 letter word SCHOOL LEAVER
I've seen this crossword clue in The Daily Telegraph
S
C
H
O
O
L
L
E
A
V
E
R
Buddy explains!
The straight part
We know that "one no longer in form" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one no longer in form = school leaver.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 SCHOOL LEAVER mean?
SCHOOL LEAVER noun- A high-school student who has recently graduated.