Protest business over medium, unflavoured "tea" (9)
I believe the answer is:
complaint
'protest' is the definition.
(I've seen this before)
'business over medium unflavoured tea' is the wordplay.
'business' becomes 'co' (short for company).
'over medium' becomes 'm' (as in UK clothing sizes. I am not sure about the 'over' bit.).
'unflavoured' becomes 'plain' (I've seen this before).
'tea' becomes 't' (I've seen this in other clues).
'co'+'m'+'plain'+'t'='COMPLAINT'
Can you help me to learn more?
(Other definitions for complaint that I've seen before include "ill-feeling" , "cause for an expression of disappointment" , "Grievance; illness" , "grouse" , "Compliant in grievance" .)