Yours truly’s resigning from tax office (8)
I believe the answer is:
position
'office' is the definition.
(position is a kind of office)
'yours truly's resigning from tax' is the wordplay.
'yours truly's' becomes 'I'm' (I'm).
'resigning from' indicates named letters should be taken away.
'tax' becomes 'imposition' (I've seen this before).
'imposition' with 'im' removed is 'POSITION'.
(Other definitions for position that I've seen before include "Attitude, job" , "Locate" , "Place where something is" , "Put" , "Set in place" .)