Absorption? Unusually large bog will absorb one (7)
I believe the answer is:
osmosis
'absorption?' is the definition.
(chemical absorption process)
'unusually large bog will absorb one' is the wordplay.
'unusually large' becomes 'OS' (short for over-size).
'bog' becomes 'moss' (Scottish term for a bog).
'will absorb' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'moss' enclosing 'i' is 'mosis'.
'os'+'mosis'='OSMOSIS'
(Other definitions for osmosis that I've seen before include "Diffusions of liquids" , "Diffusion of liquid through porous barrier" , "Large shopping area" , "gradual diffusion" , "Passage of bodily fluid through porous partition" .)