Doc writes: welcome to Smurf as our third new compiler joining our team.
 
One unclued light, the heart of which is of essence, is linked with eight other unclued lights, verifiable in Brewer. Two further unclued lights are appositely characteristic. One other light is abbreviated. Ignore an apostrophe.
 

Across

4    No can do? (11, two words)
11    A new city in Italy, southern state capital (9)
12    Soldiers resentful, not half, in revolution (5)
15    Old bus company transporting Irma from America (3, initials)
18    In a stealthy way, enticing to Grand Central (7)
22    Excessively short skirt is picked up (6, two words)
23    Vagueness thus always shown...