One watching six sheep by river (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.
'six sheep by river' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'sheep' becomes 'ewe' (a ewe is a female sheep).
'by' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation for river).
'vi'+'ewe'+'r'='VIEWER'
Can you help me to learn more?
(Other definitions for viewer that I've seen before include "One watching, eg, TV" , "Observer" , "Onlooker" , "Review for TV watcher" , "One seeing" .)