Long boring speech illustrating singular belief … (6)
I believe the answer is:
screed
'long boring speech' is the definition.
(a screed is a tedious speech or piece of writing)
'singular belief' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'belief' becomes 'creed' (creed is a kind of belief).
's'+'creed'='SCREED'
'illustrating' is the link.
(Other definitions for screed that I've seen before include "Tiresomely long letter about creeds" , "Long tedious piece of writing" , "Lengthy piece of writing" , "Layer applied to floor" , "Lengthy statement" .)