Single backing track about German with sin (8)
I believe the answer is:
enormity
'sin' is the definition.
(enormity can mean a terrible sin)
'single backing track about german with' is the wordplay.
'single' becomes 'one' (synonyms).
'backing' shows that the letters should be reversed in order.
'track' becomes 'ry'.
'about' means one lot of letters goes inside another.
'german with' becomes 'mit' (German for with).
'one' in reverse letter order is 'eno'.
'ry' enclosing 'mit' is 'rmity'.
'eno'+'rmity'='ENORMITY'
(Other definitions for enormity that I've seen before include "Outrage" , "Vastness - great crime" , "Great wickedness - more tiny (anag)" , "Outrageous act" , "Shocking act" .)