The PM's post (5)
I believe the answer is:
after
'post' is the definition.
(the prefix 'post-' means 'after')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for after that I've seen before include "Behind; in quest of" , "Looking for" , "At a later time or behind" , "Post, later than" , "Subsequent to" .)