Girl in the morning leaving the US (5)
I believe the answer is:
erica
'girl' is the definition.
(I've seen this before)
'morning leaving the us' is the wordplay.
'morning' becomes 'am'.
'leaving' suggests deleting specific letters.
'the us' becomes 'america' ('America' can be a synonym of 'us').
'america' with 'am' removed is 'ERICA'.
'in the' is the link.
(Other definitions for erica that I've seen before include "growing up on the moors" , "Woman" , "I care about Heather, oddly" , "Heathland shrub; girl's name" , "ling" .)