Percussionist finally nears disaster at the back (6)
I believe the answer is:
astern
'at the back' is the definition.
(I've seen this before)
'percussionist finally nears disaster' is the wordplay.
'finally' suggests the final letters.
'disaster' indicates anagramming the letters.
The final letter of 'percussionist' is 't'.
't'+'nears'='tnears'
'tnears' anagrammed gives 'ASTERN'.
(Other definitions for astern that I've seen before include "The wake is" , "Towards the rear of boat" , "Towards rear of a vessel" , "Nautically behind" , "at the vessel's rear" .)