So burdensome as to make one terribly sour (7)
I believe the answer is:
onerous
'so burdensome' is the definition.
'onerous' can be an answer for 'burdensome' (synonyms). I'm unsure of the 'so' bit.
'to make one terribly sour' is the wordplay.
'to make' becomes 'one' (I can't justify this - if you can you should believe this answer much more).
'one terribly' indicates an anagram.
'sour' is an anagram of 'rous'.
'one'+'rous'='ONEROUS'
'as' is the link.
Can you help me to learn more?
(Other definitions for onerous that I've seen before include "Involving a great deal of effort" , "Burdensome, oppressive" , "Burdensome, weighty" , "Burdensome, taxing" , "Oppressive, difficult to bear" .)