Took too much on at once, ultimately (7)
I believe the answer is:
overate
'took too much on at once ultimately' is the definition.
I can't tell whether this definition defines the answer.
'on at once ultimately' is the wordplay.
'on' becomes 'over' (over can mean on top of).
'ultimately' indicates one should take the final letters.
The last letter of 'once' is 'e'.
'over'+'at'+'e'='OVERATE'
Can you help me to learn more?
(Other definitions for overate that I've seen before include "consumed too much" , "got stuffed" , "Gormandised" , "Binged, glutted" .)