Teach that curt, snide characters are shown up (8)
I believe the answer is:
instruct
'teach' is the definition.
(synonyms)
'that curt snide characters are shown up' is the wordplay.
'characters' says the answer is hidden in the clue (some letters or characters from the clue).
'are shown up' says the letters should be written backwards.
'tcurtsni' is hidden in the letters of 'that curt snide'.
'tcurtsni' backwards is 'INSTRUCT'.
(Other definitions for instruct that I've seen before include "Command; teach" , "Coach" , "Give lesson or order to" , "Inform or authorise" , "Impart skills or knowledge to" .)