Answer:
Here's my breakdown:
inter - between
cess - go
sion - nominalizer (make it a noun)
related words are intersection, intervention, recession (a going back)
Here's what an online etymology site says :
http://www.etymonline.com/index.php?search=intercession&searchmode=none
intercession
from Latin c.1500 "intersessionum" : a going between
or
http://www.etymonline.com/index.php?term=intercede
Intercede
from Latin c.1570 "intercedere" : inter - "between", cedere - "go"