The most common and most likely answer to this crossword clue is the 7 letter word OSMOSIS
I've seen this crossword clue in The Times
O
S
M
O
S
I
S
More clues leading to the result OSMOSIS
- Gradual assimilation of silicon in universe, not carbon
- Slow absorption I found in very big Scottish bog
- Gradually assimilating very big second sibling
- . Way to absorb God – Sam worships regularly
- Effortless adoption thus returned second sibling
- Old Saxon lawgiver one for eastern assimilation
- Is oxygen rejected in rocky moss’s diffusion process
- After great moment, relative learning gradually
- Gradual acceptance to mind big, second sister
- Becoming absorbed by large flowerless plant seen around island
Buddy explains!

The straight part
We know that "Slow absorption" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slow absorption = osmosis.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 OSMOSIS mean?
OSMOSIS noun- The net movement of solvent molecules, usually water, from a region of lower solute concentration to a region of higher solute concentration through a partially permeable membrane.
- Picking up knowledge accidentally, without actually seeking that particular knowledge.