One million after tax (6)
I believe the answer is:
impost
'tax' is the definition.
(impost is a kind of tax)
'one million after' is the wordplay.
'one million' becomes 'im' (resembles 1m, 1 million).
'after' becomes 'post' (suffix meaning after, eg post-war).
'im'+'post'='IMPOST'
(Other definitions for impost that I've seen before include "A tax or duty levied" , "Customs duty" .)