A nut cracker Nellie is holding (6)
I believe the answer is:
kernel
'a nut' is the definition.
(I've seen this in another clue)
'cracker nellie is holding' is the wordplay.
'is holding' indicates the answer is hidden within the clue.
'KERNEL' is hidden in the letters of 'cracker nellie'.
(Other definitions for kernel that I've seen before include "Nut centre; sounds like a rank" , "Edible seed of nut" , "Heart of a nut" , "Inner part of a seed or grain" , "Hard centre of nut" .)