Unexpectedly, Bach work's unknown within northern church (2,6)
I believe the answer is:
by chance
'unexpectedly' is the definition.
(synonyms)
'bach work's unknown within northern church' is the wordplay.
'work' is an anagram indicator.
'unknown' becomes 'y' (algebra).
'within' means one lot of letters goes inside another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'church' becomes 'CE' (abbreviation for Church of England).
'bach' is an anagram of 'bcha'.
'bcha' enclosing 'y' is 'bycha'.
'bycha'+'n'+'ce'='BY CHANCE'
(Other definitions for by chance that I've seen before include "unwittingly" , "As it happened, without being planned" , "Without advance planning" , "Accidentally, not planned" , "Unplanned, as it happened" .)