Twice appears as understudy (6)
I believe the answer is:
double
This clue is a double definition.
'twice' is the first definition.
(synonyms)
'understudy' is the second definition.
(I've seen this before)
'appears as' acts as a link.
(Other definitions for double that I've seen before include "Make twice as many" , "with which darts player gets leg over?" , "Multiply by two" , "Score of forty with one dart, perhaps?" , "Second charge for the same offence" .)