Still showing sign of nerves supporting short celebrity (6)
I believe the answer is:
static
'still' is the definition.
(static can mean still or not moving)
'sign of nerves supporting short celebrity' is the wordplay.
'sign of nerves' becomes 'tic' (as in a nervous tic).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'short' means to remove the last letter.
'celebrity' becomes 'star' (a star is a celebrity or famous person).
'star' with its last letter taken off is 'sta'.
'tic' after 'sta' is 'STATIC'.
'showing' acts as a link.
(Other definitions for static that I've seen before include "problem with radio reception" , "Unmoving like kind of electricity" , "Not moving or changing" , "disturbing thing" , "fixed" .)