Unlimited time on office duty (6)
I believe the answer is:
impost
'duty' is the definition.
(I know that impost is a type of duty)
'unlimited time on office' is the wordplay.
'unlimited' indicates the central letters (discard the edges or 'limits' of the word).
'on' means one lot of letters go next to another.
'office' becomes 'post' (both can mean a position or job).
The middle of 'time' is 'im'.
'im'+'post'='IMPOST'
(Other definitions for impost that I've seen before include "Customs duty" , "A tax or duty levied" .)