Had a session, but not all day (3)
I believe the answer is:
sat
This clue could be a double definition.
'had a session' is the first definition.
(I've seen this in another clue)
'not all day' is the second definition.
'sat' can be an answer for 'day' (abbreviation for Saturday). I'm not certain of the 'not all' bit.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for sat that I've seen before include "Was in session" , "Rested" , "Was couch potato" , "Took (examination)" , "Posed (for an artist)" .)