Quins have announced that the 23-year-old will head back to Kingston Park this summer.
Clegg, who has represented England Saxons and England Under-20s, has been on loan at Newcastle's Championship rivals Leeds Carnegie since Christmas.
He left the Falcons to join Quins in 2009, making more than 50 appearances, but will now move back to Newcastle ahead of their anticipated Premiership return.
The Falcons are firm favourites to secure promotion via next month's Championship play-offs, replacing London Welsh in the Premiership next season.
- Sports & Recreation