Old moss formation is a gradual assimilation (7)
I believe the answer is:
osmosis
'a gradual assimilation' is the definition.
(I've seen this before)
'old moss formation is' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'formation' indicates an anagram.
'moss' is an anagram of 'smos'.
'o'+'smos'+'is'='OSMOSIS'
(Other definitions for osmosis that I've seen before include "Process of learning something gradually and without effort" , "Passage of bodily fluid through porous partition" , "Something like dialysis" , "Miss Soo? (anag)" , "Gradual flow" .)