How can I delete a value from a list in another a list

Question

I have created a list with three variables.

l = a,b,c

For the third variable c in the list, I created another list of values.

c = 1, 2,3,4,5

i.e. list_new = [a,b,[1,2,3,4,5]]

How can I delete the last value in c (i.e. 5, a value from a list in another list? Function?