Series of low very low grades deciding contests (7)
I believe the answer is:
Here is my best explanation:
'deciding contests' is the definition.
The definition and answer can be both acts as well as being plural nouns.
Perhaps you can see a link between them that I don't see?
'series of low very low grades' is the wordplay.
'series' becomes 'run' (run is a kind of series).
'low very low grades' becomes 'fs' (I can't justify this - if you can you should believe this answer much more).
(Another definition for runoffs that I've seen is "races to find resolution".)