Being sixteen, upset before church (9)
I believe the answer is:
existence
'being' is the definition.
('existence' can be a synonym of 'being')
'sixteen upset before church' is the wordplay.
'upset' indicates anagramming the letters.
'before' is a charade indicator (letters next to each other).
'church' becomes 'ce' (abbreviation for Church of England).
'sixteen' is an anagram of 'existen'.
'existen'+'ce'='EXISTENCE'
(Other definitions for existence that I've seen before include "Way of life" , "State of being" , "The business of being" , "A way of living" , "survival" .)