Instant but extremely tortuous doubts (6,8)
I believe the answer is:
second thoughts
'doubts' is the definition.
(I've seen this before)
'instant but extremely tortuous' is the wordplay.
'instant' becomes 'second' (synonyms).
'but' becomes 'though' (I've seen this before).
'extremely tortuous' becomes 'ts' (I can't explain this - if you can you should give a lot more credence to this answer).
'second'+'though'+'ts'='SECOND THOUGHTS'
Can you help me to learn more?
(Other definitions for second thoughts that I've seen before include "revised opinion" , "Change of mind" , "Doubts; reconsideration" , "change of heart" , "change of plan" .)