White powder tastes alarmingly like cocaine at first (4)
I believe the answer is:
talc
'white powder' is the definition.
(I've seen this before)
'tastes alarmingly like cocaine at first' is the wordplay.
'at first' indicates taking the first letters.
The initial letters of 'tastes alarmingly like cocaine' is 'TALC'.
(Other definitions for talc that I've seen before include "Powder used as toiletry" , "After-bath powder" , "Powdered mineral" , "Powdered substance" , "Bathroom powder (abbr)" .)