Cosmetic dentist's first in command (6)
I believe the answer is:
powder
'cosmetic' is the definition.
(I have seen 'Facial cosmetic ' mean 'powder' so perhaps 'cosmetic' could also mean 'powder')
'dentist's first in command' is the wordplay.
'dentist's first' becomes 'd' (pst letter of 'dentist').
'in' means one lot of letters goes inside another.
'command' becomes 'power' (I can't explain this - if you can you should believe this answer much more).
'd' placed inside 'power' is 'POWDER'.
Can you help me to learn more?
(Other definitions for powder that I've seen before include "Sort of snow" , "Solid that has been pulverised" , "Small loose particles" , "Pulverise" , "Solid substance in tiny loose particles" .)