Bitterness amiss in American pollster (6)
I believe the answer is:
gallup
'american pollster' is the definition.
The definition suggests a singular noun which matches the answer.
'bitterness amiss' is the wordplay.
'bitterness' becomes 'gall' (synonyms).
'amiss' becomes 'up' (I can't explain this - if you can you should give a lot more credence to this answer).
'gall'+'up'='GALLUP'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for gallup that I've seen before include "Opinion poll - sounds in a hurry" , "US pioneer of opinion polls" .)