Seer conjures up a rosy ethos (10)
I believe the answer is:
soothsayer
'seer' is the definition.
(I know that soothsayer is a type of seer)
'conjures up a rosy ethos' is the wordplay.
'conjures up' indicates an anagram (I've seen 'conjuring up' mean this).
'a'+'rosy'+'ethos'='arosyethos'
'arosyethos' with letters rearranged gives 'SOOTHSAYER'.
(Other definitions for soothsayer that I've seen before include "He warned Caesar" , "forecaster" , "Prophet" , "One foreseeing future" .)