For a start, half of us are in arrears (5)
I believe the answer is:
debut
'for a start' is the definition.
(debut is a kind of start)
'half of us are in arrears' is the wordplay.
'half of' means to take half the letters.
'are in' indicates putting letters inside.
'arrears' becomes 'debt' (I've seen this in other clues).
'us' with half the letters taken is 'u'.
'u' inserted inside 'debt' is 'DEBUT'.
(Other definitions for debut that I've seen before include "First appearance in society" , "entrance" , "First public appearance of performer" , "Tubed (anag.)" , "First appearence as of society girl" .)