Doctor heading off employer? Nonsense (5)
I believe the answer is:
dross
'nonsense' is the definition.
(dross can mean nonsense or rubbish)
'doctor heading off employer?' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'heading off' means to remove the first letter.
'employer?' becomes 'boss' (boss is a kind of employer).
'boss' with its first letter taken away is 'oss'.
'dr'+'oss'='DROSS'
(Other definitions for dross that I've seen before include "Worthless stuff" , "Worthless or dangerous material" , "It's Poor" , "Impurity" , "Scum on molten metal" .)