Speaker's earnings corrupt post-holder (9)
I believe the answer is:
incumbent
'post-holder' is the definition.
(the current holder of a post)
'speaker's earnings corrupt' is the wordplay.
I cannot quite understand how this works, but
'corrupt' could be 'bent' (I've seen this before) and 'bent' is found in the answer.
This explanation may well be incorrect...
(Other definitions for incumbent that I've seen before include "Mandatory" , "official?" , "Currently holding an office" , "vicar?" , "Functionary" .)