Mineral doctor holding variable amount (6)
I believe the answer is:
gypsum
'mineral doctor holding variable amount' is the definition.
I can't judge whether this defines the answer.
'mineral doctor holding variable amount' is the wordplay.
'mineral doctor' becomes 'gp' (general practitioner. I am not sure about the 'mineral' bit.).
'holding' indicates putting letters inside.
'variable' becomes 'y' (algebra).
'amount' becomes 'sum' (synonyms).
'gp' placed around 'y' is 'gyp'.
'gyp'+'sum'='GYPSUM'
(Other definitions for gypsum that I've seen before include "Ingredient of plaster of Paris" , "Plaster of Paris is made from this chalk-like mineral" , "Plaster of Paris is made from this chalk-like substance" , "Spy mug made from this white mineral" , "Base of plaster of Paris" .)