Sikh’s opening statement of belief? It’s a long document (6)
I believe the answer is:
screed
'a long document' is the definition.
(I've seen this before)
'sikh's opening statement of belief?' is the wordplay.
'opening' says to take the initial letters.
'statement of belief?' becomes 'creed' (a creed is a statement of someone's beliefs).
The first letter of 'sikhs' is 's'.
's'+'creed'='SCREED'
'it's' is the link.
(Other definitions for screed that I've seen before include "Lengthy statement" , "Layer applied to floor" , "A long written effusion" , "A long, written passage" , "Layer of mortar to level a floor" .)