Am leaving US girl (5)
I believe the answer is:
erica
'girl' is the definition.
(I've seen this before)
'am leaving us' is the wordplay.
'leaving' indicates named letters should be taken away.
'us' becomes 'america' (synonyms).
'america' with 'am' removed is 'ERICA'.
(Other definitions for erica that I've seen before include "Heather, for instance" , "Moorland plant, girl's name" , "Girl's name - heather" , "Heathland shrub; girl's name" , "Heath genus; girl's name" .)