Introduce gradually some plain Stilton (6)
I believe the answer is:
instil
'introduce gradually' is the definition.
(I've seen this before)
'some plain stilton' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'INSTIL' is hidden within 'plain stilton'.
(Other definitions for instil that I've seen before include "Inject as in list" , "Put gradually into mind" , "Inject ideas into mind" , "Gradually establish something in a person's mind" , "Implant (in the mind)" .)