Failing to start, expensive sound processing unit (3)
I believe the answer is:
ear
'sound processing unit' is the definition.
'failing to start expensive' is the wordplay.
'failing to start' means to remove the first letter.
'expensive' becomes 'dear' (I've seen this before).
'dear' with its first letter taken off is 'EAR'.
(Other definitions for ear that I've seen before include "good sense of music" , "Corn seed-holder" , "Auricle" , "Place for a plug" , "Lughole" .)