The most common and most likely answer to this crossword clue is the 8 plus 4 letter word CRITICAL MASS
I've seen this crossword clue in The Guardian
C
R
I
T
I
C
A
L
M
A
S
S
Buddy explains!
The straight part
We know that "what’s needed to sustain fission" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what’s needed to sustain fission = critical mass.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.