Overturn writer after union note (5)
I believe the answer is:
upend
'overturn' is the definition.
(to upend is to turn over)
'writer after union note' is the wordplay.
'writer' becomes 'pen' (something which writes).
'after' is a charade indicator (letters next to each other).
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'note' becomes 'd' (musical note).
'pen' put after 'u' is 'upen'.
'upen'+'d'='UPEND'
(Other definitions for upend that I've seen before include "Put into disarray" , "Turn top to bottom" , "make stand" , "Tip on side" , "Affect drastically" .)