Literary set in a punt on river in opium dream (8)
I believe the answer is:
alphabet
'literary set' is the definition.
(set of letters)
'a punt on river in opium dream' is the wordplay.
'punt' becomes 'bet' (I've seen this in other clues).
'on' says to put letters next to each other.
'river in opium dream' becomes 'Alph' (sacred river from Coleridge's poem Kubla Khan).
'a'+'bet'='abet'
'abet' after 'alph' is 'ALPHABET'.
'in' is the link.
(Other definitions for alphabet that I've seen before include "Set of letters used in writing" , "lots of characters" , "Depend (on)" , "needing help on mountain height" , "Group of characters" .)