Sign of nerves in a critical situation (3)
I believe the answer is:
tic
'sign of nerves' is the definition.
(as in a nervous tic)
'in a critical situation' is the wordplay.
'in' indicates a hidden word.
'TIC' is hidden within 'critical situation'.
(Other definitions for tic that I've seen before include "Muscular contraction" , "Spasmodic muscular twitch" , "Regular muscular twitch" , "Convulsive twitch (of face)" , "Nervous twitch" .)