Defiant leader of union advanced after brief upset (9)
I believe the answer is:
truculent
'defiant' is the definition.
('truculent' can be similar in meaning to 'defiant')
'leader of union advanced after brief upset' is the wordplay.
'leader of' indicates taking the first letters.
'advanced' becomes 'lent' (money).
'after' says to put letters next to each other.
'brief' becomes 'curt' ('curt' can be a synonym of 'brief').
'upset' says the letters should be written in reverse.
The initial letter of 'union' is 'u'.
'curt' written backwards gives 'truc'.
'u'+'lent'='ulent'
'ulent' put after 'truc' is 'TRUCULENT'.
(Other definitions for truculent that I've seen before include "Quarrelsome" , "Surly" , "Defiantly aggressive" , "Obstreperous" , "Rude and aggressive" .)