Ring everyone after onset of chickenpox (4)
I believe the answer is:
call
'ring' is the definition.
('call' can be a synonym of 'ring')
'everyone after onset of chickenpox' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'after' says to put letters next to each other.
'onset of' indicates taking the first letters.
The first letter of 'chickenpox' is 'c'.
'all' put after 'c' is 'CALL'.
(Other definitions for call that I've seen before include "Hail; visit" , "Cry - summon" , "Shout - name" , "Be in control" , "Summon - visit" .)