Defiant trade union's bid curtailed undergoing restructure (13)
I believe the answer is:
insubordinate
'defiant' is the definition.
('insubordinate' can be similar in meaning to 'defiant')
'trade union's bid curtailed undergoing restructure' is the wordplay.
'curtailed' means to remove the last letter.
'undergoing restructure' indicates an anagram.
'trade'+'unions'+'bid'='tradeunionsbid'
'tradeunionsbid' with its final letter removed is 'tradeunionsbi'.
'tradeunionsbi' is an anagram of 'INSUBORDINATE'.
(Other definitions for insubordinate that I've seen before include "Badly behaved" , "Rebellious" , "Insolent" , "Recalcitrant" , "challenging authority" .)