This question already has an answer here:
An interesting question came up last night.
Player X is 4th lvl cleric and 1st lvl monk.
Produce flame can be thrown for d8 of damage. Its damage increases to 2d8 at 5th lvl.
Does this mean at 5th lvl overall or at 5th lvl of the specific class.
We ruled that the damage won't increase until he is a 5th lvl cleric since its a cleric spell.
If there's an official ruling for this its eluding me and I apologize! ;-)