First of all, can everyone read this? Sure thing (4)
I believe the answer is:
cert
'sure thing' is the definition.
(I've seen this before)
'first of all can everyone read this?' is the wordplay.
'first of all' suggests taking the first letters.
The first letters of 'can everyone read this' is 'CERT'.
(Other definitions for cert that I've seen before include "Safe bet" , "Sure thing, in short" , "Banker" , "Inevitable event" , "Sure thing (that could be dead!)" .)