Boss first to leave plant (5)
I believe the answer is:
aster
'plant' is the definition.
(aster is a kind of plant)
'boss first to leave' is the wordplay.
'boss' becomes 'rets' (I can't explain this - if you can you should believe this answer much more).
'first' becomes 'a'.
'to leave' says the letters should be written backwards (I've seen 'left' mean this).
'rets'+'a'='retsa'
'retsa' reversed gives 'ASTER'.
Can you help me to learn more?
(Other definitions for aster that I've seen before include "Plant of Michaelmas daisy family" , "Stare at pretty flower" , "Purple pink flowered plant" , "Christmas daisy" , "Plant like a Michaelmas daisy" .)