Session (7)
'session' is the definition.
(I know that session can be written as sitting)
This is the entire clue.
(Other definitions for sitting that I've seen before include "Session of spiritualists or court" , "such an easy target" , "Well placed" , "Being portrayed" , "On a seat" .)
(Other definitions for jam that I've seen before include "Preserve - predicament" , "Spread; improvise (jazz)" , "Preserve; pack tightly" , "Fruit preserve" , "Block signals" .)