Ancient drug injected into prudish woman (8)
I believe the answer is:
primeval
'ancient' is the definition.
(I've seen this before)
'drug injected into prudish woman' is the wordplay.
'drug' becomes 'e' (E is another name for ecstasy).
'injected into' is an insertion indicator.
'prudish' becomes 'prim' (synonyms).
'woman' becomes 'val' (val is a name for a woman).
'prim'+'val'='primval'
'e' placed within 'primval' is 'PRIMEVAL'.
(Other definitions for primeval that I've seen before include "Instinctive" , "Very old" , "Prehistoric" , "From the earliest times" , "Belonging to the Earth's beginnings" .)