It's not unusual for girl getting left behind (6)
I believe the answer is:
normal
'it's not unusual' is the definition.
(I've seen this in another clue)
'girl getting left behind' is the wordplay.
I cannot quite see how this works, but
'girl' could be 'norma' and 'norma' is located in the answer.
'left' could be 'l' (common abbreviation) and 'l' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'for' acts as a link.
Can you help me to learn more?
(Other definitions for normal that I've seen before include "Unremarkable" , "Commonplace" , "Average or ordinary" , "it's what you'd expect" , "Customary" .)