I say you in entering sour problem, are quite calamitous (7)
I believe the answer is:
ruinous
'quite calamitous' is the definition.
'ruinous' can be an answer for 'calamitous' (synonyms). I'm not sure about the 'quite' bit.
'i say you in entering sour problem' is the wordplay.
'i say you' becomes 'u' (I can't explain this - if you can you should give a lot more credence to this answer).
'entering' is an insertion indicator.
'problem' indicates an anagram.
'u'+'in'='uin'
'sour' is an anagram of 'rous'.
'uin' going inside 'rous' is 'RUINOUS'.
'are' acts as a link.
Can you help me to learn more?
(Other definitions for ruinous that I've seen before include "far too expensive" , "Devastating - dilapidated" , "causing harm?" , "Calamitous, crippling" , "Tumbledown - expensive" .)