Writer sitting at foot of tree is quite bitter (7)
I believe the answer is:
acerbic
'bitter' is the definition.
(synonyms)
'writer sitting at foot of tree is quite' is the wordplay.
I cannot quite see how this works, but
'writer' could be 'bic' (kind of pen) and 'bic' is located in the answer.
'tree' could be 'acer' (acer is a type of tree) and 'acer' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for acerbic that I've seen before include "Astringent; sour" , "Bitter in taste -- harsh in tone" , "Sour; sharp" , "Biting" , "Astringent in tone" .)