Mineral from isthmus pygmies sent north (6)
I believe the answer is:
gypsum
'mineral' is the definition.
(gypsum is a mineral)
'from isthmus pygmies sent north' is the wordplay.
'from' says the answer is hidden in the clue.
'north' is a reversal indicator (in a down clue, letters go up or 'north').
'muspyg' can be found hidden inside 'isthmus pygmies sent'.
'muspyg' back-to-front is 'GYPSUM'.
(Other definitions for gypsum that I've seen before include "Chalky material in plaster of Paris" , "Alabaster" , "Mineral used to make plaster of Paris" , "Chalky mineral from which plaster of Paris is made" , "Mineral used in making plaster of Paris" .)