Broadcast set to receive unknown viewer's complaint (4)
I believe the answer is:
stye
'viewer's complaint' is the definition.
(I've seen this before)
'broadcast set to receive unknown' is the wordplay.
'broadcast' indicates anagramming the letters (letters scattered or cast around).
'to receive' is an insertion indicator.
'unknown' becomes 'y' (algebra).
'set' anagrammed gives 'ste'.
'ste' placed around 'y' is 'STYE'.
(Other definitions for stye that I've seen before include "Tamworth residence" , "producing eyesore" , "Red eye" , "Inflamed swelling on the edge of an eyelid" , "pain for the observer" .)