Boss, heading off for plant (5)
I believe the answer is:
aster
'plant' is the definition.
(aster is a kind of plant)
'boss heading off' is the wordplay.
'boss' becomes 'master' (I have seen 'The boss' mean 'master' so perhaps 'boss' could also mean 'master').
'heading off' means to remove the first letter.
'master' with its first letter taken away is 'ASTER'.
'for' acts as a link.
(Other definitions for aster that I've seen before include "Plant like a Michaelmas daisy" , "Garden plant" , "Tears about bright flower" , "eg Michaelmas daisy" , "Stare (anag.)" .)