Girl before noon leaving US (5)
I believe the answer is:
erica
'girl' is the definition.
'before noon leaving us' is the wordplay.
'before noon' becomes 'am' (as in AM on a clock).
'leaving' is a deletion indicator.
'us' becomes 'America' (synonyms).
'america' with 'am' taken out is 'ERICA'.
(Other definitions for erica that I've seen before include "I care about Heather, oddly" , "Plant genus of garden heather" , "Shrub genus; girl's name" , "Heather, for instance" , "I care (anag) - plant - girl" .)