Stealthy new Tories stir up unions at first (13)
I believe the answer is:
surreptitious
'stealthy' is the definition.
(synonyms)
'new tories stir up unions at first' is the wordplay.
'new' is an anagram indicator.
'at first' indicates taking the first letters.
The first letter of 'unions' is 'u'.
'tories'+'stir'+'up'+'u'='toriesstirupu'
'toriesstirupu' with letters rearranged gives 'SURREPTITIOUS'.
(Other definitions for surreptitious that I've seen before include "Done in secret; stealthy" , "Sneaky" , "Done covertly" , "Covert, clandestine" , "Furtive" .)