From Discworld & Terry Pratchett Wiki
A human constable in the Ankh-Morpork City Watch, Fiddyment appears in The Truth. When the steeple-jack Arthur Crank disrupts traffic in Artificer Street by threatening to jump off a high building ledge, Fiddyment is the officer on the spot who must do something about it. Given the good citizens approach to interesting street theater, many carts have stopped and a large mob has gathered to watch what happens.
While William de Worde climbs up onto the ledge with Mr Crank to interview him for the Ankh-Morpork Times, discovering in the process that the man does not actually intend to jump off, just to threaten to jump until his demands are met (about AM$20 should do it). Fiddyment instead fetches the man's wife (to whom Arthur has been happily married for three years, over three decades ago) who shouts at her husband, and threatens to take away his trousers if he doesn't come down right now.
Fiddyment appears later in the story, when the watch are required to secure the Patrician's Palace after it is reported that the Patrician has stabbed one of his clerks. After Sergeant Detritus is confronted and verbally bested at the gates by William de Worde, he gets Fiddyment to escort William and Otto Chriek to Commander Vimes with the instruction "dey not to fall down any steps or nuffin".