Farewell notice that is given to union leader (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
'notice that is given to union leader' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'given to' means one lot of letters go next to another.
'leader' suggests taking the first letters.
The initial letter of 'union' is 'u'.
'ad'+'ie'+'u'='ADIEU'
(Other definitions for adieu that I've seen before include "Auf wiedersehen" , "So long abroad" , "Goodbye, mademoiselle" , "Some say it when leaving" , "the last word in France" .)