Son in club concert (6)
I believe the answer is:
unison
'concert' is the definition.
(to work in concert is to work in unison)
'son in club' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'in' means one lot of letters goes inside another.
'club' becomes 'union' (workers' club).
's' placed into 'union' is 'UNISON'.
(Other definitions for unison that I've seen before include "In agreement - like sound of choir" , "As one, singers perhaps" , "Agreement or concord, as with choir" , "Type of singing" , "A coincidence of musical notes" .)