Extremely hard, yes, said to be hard to get the pigment (8)
I believe the answer is:
dyestuff
'the pigment' is the definition.
(thesaurus)
'extremely hard yes said to be hard' is the wordplay.
'extremely hard yes' becomes 'dyes' (I can't justify this - if you can you should give a lot more credence to this answer).
'said to be' shows a homophone (sound like).
'hard' becomes 'tough' (similar in meaning).
'tough' sounds like 'tuff'.
'dyes'+'tuff'='DYESTUFF'
'to get' is the link.
Can you help me to learn more?
(Other definitions for dyestuff that I've seen before include "Colouring agent" , "Colourant" , "Substance used in staining" , "Anil perhaps" , "Colouring substance" .)