Instant cure-all, possibly composed of tiny pieces (9)
I believe the answer is:
molecular
'composed of tiny pieces' is the definition.
'instant cure-all possibly' is the wordplay.
'instant' becomes 'mo'.
'possibly' is an anagram indicator.
'cure'+'all'='cureall'
'cureall' anagrammed gives 'lecular'.
'mo'+'lecular'='MOLECULAR'
(Other definitions for molecular that I've seen before include "Relating to groups of atoms bonded together" , "Dealing with small particles" , "Relating to the smallest particles of a substance" .)