Diatribe from barman taking grass (6)
I believe the answer is:
screed
'diatribe' is the definition.
(I've seen this in another clue)
'barman taking grass' is the wordplay.
'barman' becomes 'sc' (I've seen this in other clues).
'taking' says to put letters next to each other.
'grass' becomes 'reed' (I've seen this before).
'sc'+'reed'='SCREED'
'from' acts as a link.
(Other definitions for screed that I've seen before include "Tiresomely long document about creeds" , "Tedious speech; cement" , "Long speech made by Saint Faith?" , "Thin layer of mortar" , "extensive text" .)