Girl left behind as usual (6)
I believe the answer is:
normal
'as usual' is the definition.
(I've seen this before)
'girl left behind' is the wordplay.
'girl' becomes 'norma' (**).
'left' becomes 'l' (common abbreviation).
'behind' means one lot of letters go next to another.
'norma'+'l'='NORMAL'
(Other definitions for normal that I've seen before include "Standard; at right angles" , "Average or ordinary" , "it's what you'd expect" , "Usual, typical, expected" , "Ordinary, usual" .)