Sixteen converts taking on church life (9)
I believe the answer is:
existence
'life' is the definition.
(I've seen this before)
'sixteen converts taking on church' is the wordplay.
'converts' is an anagram indicator.
'taking on' says to put letters next to each other.
'church' becomes 'ce' (abbreviation for Church of England).
'sixteen' anagrammed gives 'existen'.
'existen'+'ce'='EXISTENCE'
(Other definitions for existence that I've seen before include "A way of living" , "Way of life" , "survival" , "State of being" , "The business of being" .)