A shade hard, ultimately? Certainly difficult, we hear (8)
I believe the answer is:
dyestuff
'a shade' is the definition.
(substance used for its dye or shade)
'certainly difficult we hear' is the wordplay.
'certainly' becomes 'dyes' (I am not sure about this - if you are sure you should believe this answer much more).
'difficult' becomes 'tough' (similar in meaning).
'we hear' indicates a 'sounds like' (homophone) clue.
'tough' is a homophone of 'tuff'.
'dyes'+'tuff'='DYESTUFF'
'hard ultimately?' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
(Other definitions for dyestuff that I've seen before include "Colouring substance" , "Colouring agent" , "Anil perhaps" , "Colourant" , "Substance used in staining" .)