Re-arrest union leader's corrupt financial officer (9)
I believe the answer is:
treasurer
'financial officer' is the definition.
('treasurer' can be a synonym of 'financial officer')
're-arrest union leader's corrupt' is the wordplay.
'leader' suggests taking the first letters.
'corrupt' indicates anagramming the letters.
The initial letter of 'union' is 'u'.
're'+'arrest'+'u'='rearrestu'
'rearrestu' with letters rearranged gives 'TREASURER'.
(Other definitions for treasurer that I've seen before include "Society official" , "position in society?" , "Finance officer" , "Official in charge of funds" , "one safeguarding funds" .)