A nut cracker Nellie is holding (6)
I believe the answer is:
kernel
'a nut' is the definition.
(I've seen this before)
'cracker nellie is holding' is the wordplay.
'is holding' indicates a hidden word.
'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" .)