It's on the bar - that provides a challenge (3,6)
I believe the answer is:
ice bucket
'bar that provides a challenge' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'it's on the' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ice bucket that I've seen before include "To get cooler" , "Holder of frozen water" , "What waiter brings" , "Wine chiller" , "Wine cooler" .)