Article to post assuming costs primarily go up (6)
I believe the answer is:
ascend
'go up' is the definition.
(I know that go up can be written as ascend)
'article to post assuming costs primarily' is the wordplay.
'article' becomes 'a' (English indefinite article).
'to post' becomes 'send' (synonyms).
'assuming' is an insertion indicator (inserted letters are taken in or assumed).
'primarily' indicates taking the first letters.
The initial letter of 'costs' is 'c'.
'send' placed around 'c' is 'scend'.
'a'+'scend'='ASCEND'
(Other definitions for ascend that I've seen before include "Move upwards" , "shift to a higher plane" , "Come up" , "Take off" , "Dances (anag.)" .)