Is this the first single-engine helicopter clean-sheet design since 1976?
Most modern helicopters use two engines for added safety. This reduces the field of competing designs. Also, a good engineer will always leave proven details unchanged and focus on improving flawed designs. So it is hard to prove that a new design from an established helicopter company will truly be a clean-sheet design.

The possible new designs from 1976 could be:


Aerospatiale AS-350 Écureuil: First flight 1974, introduced 1975. Note that the AS-355 is a two-turbine variant of the single-turbine AS-350.
Robinson R-22: First flight 1975, introduced 1979. It has a single reciprocating engine and was indeed a clean-sheet design by Frank Robinson who started the company in 1973. He had worked previously at Bell Helicopter and Hughes Helicopters.