Cleric domain spells are extra prepared spells. The acolyte gets an extra 1st level spell slot.
ahh...let me clarify...one should assume the extra slot is for casting a domain spell (undetermined and not in the MM listing so using rule 0 here). So, for example, and Acolyte of Pelor (light domain) would have 3 prepared spells (1 + 2 for wisdom) from the cleric list and 3 1st level slots with 2 regular slots (for casting from prepared list) and 1 slot for casting burning hands (domain spell) only.
again, DM rule 0...anomaly still exists in RAW.