Old and prudish, oddly enviable (8)
I believe the answer is:
primeval
'old' is the definition.
(thesaurus)
'prudish oddly enviable' is the wordplay.
'prudish' becomes 'prim' (synonyms).
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
The alternate letters of 'enviable' are 'eval'.
'prim'+'eval'='PRIMEVAL'
'and' acts as a link.
(Other definitions for primeval that I've seen before include "From the earliest times" , "Belonging to the Earth's beginnings" , "Instinctive" , "Very old" , "having come first in history" .)