The most common and most likely answer to this crossword clue is the 6 letter word AFFLUX
A
F
F
L
U
X
Buddy explains!

The straight part
We know that "accession" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case accession = afflux.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 AFFLUX mean?
AFFLUX noun- An upward rush of fluid.
- The rise in water level (above normal) on the upstream side of a bridge or obstruction caused when the effective flow area at the obstruction is less than the natural width of the stream immediately upstream of the obstruction.