One trapped in large marsh for slow absorption (7)
I believe the answer is:
osmosis
'slow absorption' is the definition.
'osmosis' can be an answer for 'absorption' (chemical absorption process). I'm not certain of the 'slow' bit.
'one trapped in large marsh' is the wordplay.
'one' becomes 'i' (Roman numeral).
'trapped in' means one lot of letters goes inside another.
'large' becomes 'os' (abbreviation for oversize).
'marsh' becomes 'moss' (I can't justify this - if you can you should believe this answer much more).
'os'+'moss'='osmoss'
'i' inserted within 'osmoss' is 'OSMOSIS'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for osmosis that I've seen before include "Diffusion; assimilation" , "Diffusion of liquid through porous membrane" , "Diffusion through a semi-permeable membrane" , "Process of gradual assimilation of ideas" , "gradual diffusion" .)