Yes, it is indeed a short form of éclairage used on some Simca dashboards.

Here an Ariane 4 SL 1959 model dashboard:

and here is an excerpt from an add of the Vedette V8 1956 model:

The buttons labels are:
– CHAUF : Chauffage (heater) on the Vedette V8
– CLIMAT : Climatisation (air conditioning) on the Ariane 4
– START : Starter (choke)
– ECLAIR: Éclairage (lights)
– AVERT : Avertisseur (horn)
– ESS GL: Essuie glace (wiper)
– DEMAR : Démarreur (starter)

Photo credits: https://simca67etanciennes.jimdo.com/simc-adh%C3%A9rents/patrick/ and http://www.auto-pub.net/Simca/Simca_Vedette_56/Simca_Vedette_6.jpg