Second cure-all made up of groups of atoms (9)
I believe the answer is:
molecular
'of groups of atoms' is the definition.
(relating to molecules)
'second cure-all made up' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'made up' indicates an anagram.
'cure'+'all'='cureall'
'cureall' is an anagram of 'lecular'.
'mo'+'lecular'='MOLECULAR'
(Other definitions for molecular that I've seen before include "Relating to groups of atoms bonded together" , "Relating to the smallest particles of a substance" , "Dealing with small particles" .)