Short trip that is taken after class (6)
I believe the answer is:
sortie
'short trip' is the definition.
(I have seen 'short trip' mean 'sortie' so perhaps 'trip' could also mean 'sortie'; 'short' asks for a short form)
'that is taken after class' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'taken after' means one lot of letters go next to another.
'class' becomes 'sort' (synonyms).
'ie' after 'sort' is 'SORTIE'.
(Other definitions for sortie that I've seen before include "Single military flight" , "Warplane's flight" , "Raid - short trip" , "(Siege) breakout" , "Short trip; attack" .)