Many a footballer's error will haunt him, following (3)
I believe the answer is:
dog
'error will haunt him following' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'many a footballer's' is the wordplay.
'many' becomes 'd'.
'a footballer's' becomes 'og' (I can't explain this - if you can you should believe this answer much more).
'd'+'og'='DOG'
Can you help me to learn more?
(Other definitions for dog that I've seen before include "plague?" , "Worry" , "'Every . . . has its day'" , "A pet; its life prov. wretched" , "'You cannot teach an old ... new tricks'" .)