One watching pitcher going after six (6)
I believe the answer is:
viewer
'one watching' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'pitcher going after six' is the wordplay.
'pitcher' becomes 'ewer' (synonyms).
'going after' means one lot of letters go next to another.
'six' becomes 'VI' (Roman numerals).
'ewer' after 'vi' is 'VIEWER'.
Can you help me to learn more?
(Other definitions for viewer that I've seen before include "One who watches, say, TV" , "One watching, eg, TV" , "Review for TV watcher" , "He surveys, or watches TV" , "Spectator" .)