So much longer! (9)
I believe the answer is:
therefore
'so' is the definition.
(I know that so can be written as therefore)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for therefore that I've seen before include "has a triangular formation in maths?" , "It follows" , "Thus" , "In consequence" , "Ergo" .)