Popular paper by leaderless authority's beyond words (13)
I believe the answer is:
inexpressible
'beyond words' is the definition.
(I've seen this before)
'popular paper by leaderless authority's' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'paper' becomes 'express' (The Daily Express newspaper).
'by' is a charade indicator (letters next to each other).
'leaderless' means to remove the first letter.
'authority' becomes 'bible' (bible can mean any important or authoritative text).
'bible' with its initial letter taken away is 'ible'.
'in'+'express'+'ible'='INEXPRESSIBLE'
(Other definitions for inexpressible that I've seen before include "Beyond words" , "Beyond description" .)