Heather in the morning to leave the US (5)
I believe the answer is:
erica
'heather' is the definition.
(the plant genus Erica includes heather plants)
'in the morning to leave the us' is the wordplay.
'in the morning' becomes 'am' (AM on a clock).
'to leave' indicates named letters should be taken away.
'the us' becomes 'America' (synonyms).
'america' with 'am' taken away is 'ERICA'.
(Other definitions for erica that I've seen before include "I care (anag) - plant - girl" , "Shrubs" , "Moorland plant, girl's name" , "growing up on the moors" , "Heather, for instance" .)