Live in endless childhood? Certainly! (3,3)
I believe the answer is:
you bet
'certainly' is the definition.
(I've seen this before)
'live in endless childhood?' is the wordplay.
'live' becomes 'be' (synonyms).
'in' means one lot of letters goes inside another.
'endless' means to remove the last letter.
'childhood?' becomes 'youth'.
'youth' with its last letter removed is 'yout'.
'be' going inside 'yout' is 'YOU BET'.
(Other definitions for you bet that I've seen before include "Absolutely" , "Certainly! (sl.)" , "You may be sure - gamble on it" , "I'll say!" , "It's a certainty - put wager on it" .)