It's socially acceptable to be in debt at the start (5)
I believe the answer is:
debut
'the start' is the definition.
(debut is a kind of start)
'it's socially acceptable to be in debt' is the wordplay.
'it's socially acceptable' becomes 'U' (as in U and non-U language).
'to be in' is an insertion indicator.
'u' going within 'debt' is 'DEBUT'.
'at' acts as a link.
(Other definitions for debut that I've seen before include "First appearence as of society girl" , "Beginning, first performance" , "First appearance in society" , "Initial appearance" , "entrance" .)