Mineral content in stale cake, just the very first piece (4)
I believe the answer is:
talc
'mineral' is the definition.
(I know that talc is a type of mineral)
'content in stale cake just the very first piece' is the wordplay.
'content in' means to look at the middle letters.
'just the very first piece' says to take the initial letters.
The middle of 'stale' is 'tal'.
The initial letter of 'cake' is 'c'.
'tal'+'c'='TALC'
(Other definitions for talc that I've seen before include "Scented powder used after bathing" , "Softest substance in Mohs scale of hardness" , "After-bath powder" , "Drier" , "Fragrant powder" .)