Add to trade union quietly (3,2)
I believe the answer is:
tot up
'add' is the definition.
(I know that add can be written as tot up)
'to trade union quietly' is the wordplay.
'trade union' becomes 'TU' (abbreviation for trade union).
'quietly' becomes 'p' (p for piano and quiet).
'to'+'tu'+'p'='TOT UP'
(Other definitions for tot up that I've seen before include "Reckon" , "Add together (driving offences)" , "Tally" , "Count" , "Summate" .)