Contact everyone following onset of Covid (4)
I believe the answer is:
call
'contact' is the definition.
'everyone following onset of covid' is the wordplay.
'everyone' becomes 'all' (all people).
'following' says to put letters next to each other.
'onset of' suggests taking the first letters.
The first letter of 'covid' is 'c'.
'all' after 'c' is 'CALL'.
(Other definitions for call that I've seen before include "That is enough" , "Pay a visit" , "Visit; shout" , "Be in control" , "Short visit" .)