Cunningly stifle the very thing (6)
I believe the answer is:
itself
'the very thing' is the definition.
(I've seen this before)
'cunningly stifle' is the wordplay.
'cunningly' indicates an anagram (a cunning or tricky form of the letters).
'stifle' is an anagram of 'ITSELF'.
(Other definitions for itself that I've seen before include "That's it, emphatically" , "Reflexive form of an inanimate third person pronoun" , "alone by this" , "Reflexive word - stifle (anag)" , "Emphatic form of it" .)