Weaken purchasing very adult material (8)
I believe the answer is:
relevant
'material' is the definition.
(similar in meaning)
'weaken purchasing very adult' is the wordplay.
'weaken' becomes 'relent' (relent can mean to weaken or decrease in intensity).
'purchasing' means one lot of letters goes inside another (some letters take possession of others).
'very' becomes 'v' (abbreviation).
'adult' becomes 'a' (abbreviation).
'v'+'a'='va'
'relent' enclosing 'va' is 'RELEVANT'.
(Other definitions for relevant that I've seen before include "Germane or pertinent" , "Material" , "To the point" , "Apt" , "Closely connected (to the matter in hand)" .)