Reached a maximum (6)
I believe the answer is:
peaked
'reached a maximum' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for peaked that I've seen before include "Reached top condition - on mountain top?" , "Reached climax or culmination" , "Reached the height of one's powers" , "gave best performance" , "Reached highest point, say of fitness" .)