Sign of nerves in a politician (3)
I believe the answer is:
tic
'sign of nerves' is the definition.
(as in a nervous tic)
'in a politician' is the wordplay.
'in' indicates a hidden word.
'TIC' is hidden in the letters of 'politician'.
(Other definitions for tic that I've seen before include "Regular muscular twitch" , "Muscle twitch" , "Involuntary muscular spasm" , "Nervous movement" , "Facial twitch" .)