Sign Mussolini, megalomaniac, somewhat taken aback (6)
I believe the answer is:
gemini
'sign' is the definition.
(I know that Gemini is a sign)
'mussolini megalomaniac somewhat taken aback' is the wordplay.
'somewhat' indicates a hidden word.
'taken aback' says the letters should be written in reverse.
'inimeg' is hidden within 'mussolini megalomaniac'.
'inimeg' reversed gives 'GEMINI'.
(Other definitions for gemini that I've seen before include "Twins constellation" , "Heavenly sign of the Twins" , "star group" , "space programme" , "lot of stars" .)