Such poor reasoning in letter (8)
I believe the answer is:
circular
This clue is a double definition.
'such poor reasoning' is the first definition.
(circular reasoning is illogical)
'letter' is the second definition.
(type of advertisement)
'in' acts as a link.
(Other definitions for circular that I've seen before include "Widely-distributed notice" , "Perfectly round" , "plug, perhaps" , "Leaflet for mass distribution" , "Round kind of bulletin?" .)