Had a session one day (3)
I believe the answer is:
sat
This clue is a double definition.
'had a session' is the first definition.
(I've seen this in another clue)
'one day' is the second definition.
(short for Saturday)
(Other definitions for sat that I've seen before include "Took (an exam)" , "Was couch potato" , "Rested" , "Relaxed" , "Took a seat and posed for a portrait" .)