The way of one who fills a post (6)
I believe the answer is:
manner
'the way of' is the definition.
(thesaurus)
'one who fills a post' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for manner that I've seen before include "Style, way" , "Style, air" , "Way of acting or behaving" , "Kind, sort" , "Mode" .)