Layman in a primeval city imbibing milky infusion ... (7)
I believe the answer is:
amateur
'layman' is the definition.
(thesaurus)
'a primeval city imbibing milky infusion' is the wordplay.
'primeval city' becomes 'ur' (ancient city).
'imbibing' indicates putting letters inside.
'milky infusion' becomes 'mate' (South American hot drink).
'a'+'ur'='aur'
'aur' placed around 'mate' is 'AMATEUR'.
'in' is the link.
(Other definitions for amateur that I've seen before include "Person who does something as a pastime rather than a profession" , "Unprofessional" , "Tum area (anag.)" , "Inept, non-professional" , "incompetent" .)