Substance is held between ends of gadget (4)
I believe the answer is:
gist
'substance' is the definition.
(I know that substance can be written as gist)
'is held between ends of gadget' is the wordplay.
'held between' indicates putting letters inside.
'ends of' means to remove the middle letters.
'gadget' with its centre taken out is 'gt'.
'is' placed within 'gt' is 'GIST'.
(Other definitions for gist that I've seen before include "Main point of the matter" , "Substance of an argument" , "General meaning or main point" , "Central idea" , "Essential point or meaning of, say, the matter" .)