Happen to everyone following February's brief upturn (6)

'happen' is the definition.
(I know that happen can be written as befall)

'to everyone following february's brief upturn' is the wordplay.
'to everyone' becomes 'all' (all people).
'following' means one lot of letters go next to another.
'february's' becomes 'Feb's' ('Feb' can be a synonym of 'february').
'brief' means to remove the last letter (a shorter or briefer version of the word).
'upturn' says the letters should be written backwards (in a down clue, letters go up).
'febs' with its final letter taken away is 'feb'.
'feb' reversed gives 'bef'.
'all' after 'bef' is 'BEFALL'.