Accountant -- the ultimate in economy of hearing (8)
I believe the answer is:
auditory
'of hearing' is the definition.
(I've seen this before)
'accountant the ultimate in economy' is the wordplay.
'accountant' becomes 'auditor' (auditor is a kind of accountant).
'the ultimate in' says to take the final letters.
The final letter of 'economy' is 'y'.
'auditor'+'y'='AUDITORY'
(Other definitions for auditory that I've seen before include "as one hears" , "Relating to hearing" , "Related to the sense of hearing" , "Via the ears" .)