Fancy decaff tea — not one that's pretentious (8)
I believe the answer is:
affected
'pretentious' is the definition.
(I've seen this before)
'fancy decaff tea not one' is the wordplay.
'fancy' indicates an anagram.
'not' indicates named letters should be taken away.
'one' becomes 'a' (a thing is one thing).
'decaff'+'tea'='decafftea'
'decafftea' with 'a' taken out is 'decfftea'.
'decfftea' with letters rearranged gives 'AFFECTED'.
'that's' acts as a link.
(Other definitions for affected that I've seen before include "Putting on airs, being emotionally moved" , "in a pretentious way" , "Pretended; artificial" , "Not natural" , "Influenced; pretentious" .)