Keen viewer having a right to enter box used by commentators (6)
I believe the answer is:
larynx
'box used by commentators' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'keen viewer having a right to enter' is the wordplay.
'keen viewer' becomes 'lynx' (I can't justify this - if you can you should believe this answer much more).
'having' means one lot of letters goes inside another.
'right' becomes 'r' (abbreviation).
'to enter' means one lot of letters goes inside another.
'lynx' going around 'a' is 'laynx'.
'laynx' enclosing 'r' is 'LARYNX'.
Can you help me to learn more?
(Other definitions for larynx that I've seen before include "Human voice box" , "Where growls come from" , "Organ containing the vocal cords" , "Source of squeaks" , "Upper part of the windpipe" .)