Formerly grotesque mammoth's no show (5)
I believe the answer is:
antic
'formerly grotesque' is the definition.
'antic' can be an answer for 'grotesque' (synonyms). I am not certain of the 'formerly' bit.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for antic that I've seen before include "Prank" , "Playful caper" , "seen to be bizarre, at one time" , "Absurd action" , "Eccentric once" .)