Beef optimal: can emptied out (9)
I believe the answer is:
complaint
'beef' is the definition.
(I know that beef can be written as complaint)
'optimal can emptied out' is the wordplay.
'emptied' means to remove the middle letters.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'can' with its middle taken out is 'cn'.
'optimal'+'cn'='optimalcn'
'optimalcn' is an anagram of 'COMPLAINT'.
(Other definitions for complaint that I've seen before include "A disease" , "Indisposition" , "Compliant in grievance" , "Sickness" , "Objection" .)