Long to receive explanation for viewer's complaint (7)
I believe the answer is:
pinkeye
'viewer's complaint' is the definition.
'pinkeye' can be an answer for 'complaint' (I've seen this before). I am not sure about the 'viewer's' bit.
'long to receive explanation' is the wordplay.
'long' becomes 'pine' ('pine' can be a synonym of 'long').
'to receive' means one lot of letters goes inside another.
'explanation' becomes 'key' (key is a kind of explanation).
'pine' enclosing 'key' is 'PINKEYE'.
'for' is the link.
Can you help me to learn more?
(Other definitions for pinkeye that I've seen before include "Conjunctivitis" , "complaint" .)