Calder regular Tackleberry is the potential upsetter in the field of seven.
From the orlandosentinel.com
Shrimpy, the beatific upsetter causes trouble that the straight-man Paul must correct.
From the time.com
We could have expected Philip roth or some upsetter like Jonathan Franzen or Jonathan Lethem.
From the guardian.co.uk
Quest could be the upsetter if the budget can stretch to a chance outside the two obvious favourites.
From the nzherald.co.nz
More examples
An unexpected winner; someone who defeats the favorite competitor
Upsetter Records was a Jamaican record label set up by Lee "Scratch" Perry in 1968. Perry also opened the Upsetter Record Shop where he sold the records he produced.
The Upsetter is an album of Lee "Scratch" Perry productions, released in 1969. The album largely comprises instrumentals from Perry's studio band The Upsetters, but also features vocal tracks by Busty Brown and The Muskyteers (aka the Silvertones). ...
A horizontal forging machine where the workpiece is gripped between two grooved dies and deformed by a punch that exerts force on the end of the stock.
A machine with horizontal action, used for making upset forgings.