Had a session in a serious atmosphere (3)
I believe the answer is:
sat
'had a session' is the definition.
(I've seen this in another clue)
'in a serious atmosphere' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SAT' is hidden within 'serious atmosphere'.
(Other definitions for sat that I've seen before include "nearly half a day" , "Took a chair" , "Relaxed" , "Took (examination)" , "Took a seat and posed for a portrait" .)