Rock group in church regarding unknown service (8)
I believe the answer is:
ceremony
'service' is the definition.
(I've seen this before)
'rock group in church regarding unknown' is the wordplay.
'rock group' becomes 'REM' (REM are a US rock band).
'in' means one lot of letters goes inside another.
'church' becomes 'ce' (abbreviation for Church of England).
'regarding' becomes 'on' (on or regarding a particular topic).
'unknown' becomes 'y' (algebra).
'ce'+'on'='ceon'
'rem' inserted within 'ceon' is 'ceremon'.
'ceremon'+'y'='CEREMONY'
(Other definitions for ceremony that I've seen before include "Rite, ritual or formal act" , "the formal stand on?" , "Possibly a parade" , "Ritualised public occasion" , "Formal service" .)