Assuming Post Office finding location (8)
I believe the answer is:
positing
'assuming' is the definition.
(I've seen this before)
'post office finding location' is the wordplay.
'post office' becomes 'po' (abbreviation).
'finding' says to put letters next to each other.
'location' becomes 'siting' (where it is sited).
'po'+'siting'='POSITING'
(Other definitions for positing that I've seen before include "Postulating" , "assuming" .)