Make known it's not provided before end of May (6)
I believe the answer is:
notify
'make known' is the definition.
(I've seen this before)
'not provided before end of may' is the wordplay.
'provided' becomes 'if'.
'before' is a charade indicator (letters next to each other).
'end of' indicates one should take the final letters.
The final letter of 'may' is 'y'.
'not'+'if'+'y'='NOTIFY'
'it's' acts as a link.
(Other definitions for notify that I've seen before include "Advise, apprise" , "Formally inform" , "'Advise, tell about (6)'" , "Formally advise" , "Send word" .)