Bad mistake to have ambition (3,4)
I believe the answer is:
own goal
I believe this clue is a double definition.
'bad mistake' is the first definition.
(I've seen this before)
'have ambition' is the second definition.
(I've seen this before)
'to' acts as a link.
(Other definitions for own goal that I've seen before include "One may get back embarrassed" , "hitting the wrong one!" , "action that backfired?" , "Act harming the perpetrator's interests" , "Action taken which results in a disadvantage to the taker" .)