He's ratty when interrupted by a teetotaller (3)
I believe the answer is:
ray
'he's' is the definition.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ray that I've seen before include "may be electric" , "Man who engaged in surrealist photography" , "Second note for the flat fish" , "Beam; flat-bodied fish" , "Shaft, beam" .)