Finally we can't forget 'Carry On' with aunt's bottle exploding (4,3,3,5)
I believe the answer is:
last but not least
'finally' is the definition.
(I've seen this before)
'carry on with aunt's bottle exploding' is the wordplay.
'carry on' becomes 'last'.
'with' is an insertion indicator.
'exploding' is an anagram indicator.
'aunts'+'bottle'='auntsbottle'
'auntsbottle' is an anagram of 'tbutnotleas'.
'last' going around 'tbutnotleas' is 'LAST BUT NOT LEAST'.
'we can't forget' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for last but not least that I've seen before include "arranged in ascending order of height?" , "not applicable to descending series" , "Something not to be overlooked" , "Belying his position" , "still worth a mention?" .)