Contact everyone after onset of cyclone (4)
I believe the answer is:
call
'contact' is the definition.
'everyone after onset of cyclone' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'after' says to put letters next to each other.
'onset of' suggests taking the first letters.
The first letter of 'cyclone' is 'c'.
'all' put after 'c' is 'CALL'.
(Other definitions for call that I've seen before include "That is enough" , "Shout - name" , "decision" , "Summon, ring" , "Telephone - give a name" .)