Unopened one is left to tip over (2-3)
I believe the answer is:
up-end
'unopened one is left' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'tip over' is the wordplay.
I cannot quite understand how this works, but
'tip' could be 'end' (tip can mean the end of something) and 'end' is found in the answer.
'over' could be 'up' (e.g. if your time is up it is over) and 'up' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for up-end that I've seen before include "Turn from top to bottom" , "Put into disarray" , "Have unsettling effect on" , "Turn on its head" .)