Tax situation Imogen's on top of (6)
I believe the answer is:
'tax' is the definition.
(I know that impost is a type of duty)
'situation imogen's on top of' is the wordplay.
'situation' becomes 'post' (both can mean a job or position).
'imogen's' becomes 'I'm' (where the crossword setter is 'Imogen').
'on top of' means one lot of letters go next to another.
'post' after 'im' is 'IMPOST'.
(Other definitions for impost that I've seen before include "Customs duty" , "A tax or duty levied" .)