Trade union in retreat reversed custom (6)
I believe the answer is:
ritual
'custom' is the definition.
(I know that ritual is a type of custom)
'trade union in retreat reversed' is the wordplay.
'trade union' becomes 'TU' (abbreviation for trade union).
'in' means one lot of letters goes inside another.
'retreat' becomes 'lair' (both can mean a private hideout).
'reversed' shows that the letters should be reversed in order.
'lair' reversed gives 'rial'.
'tu' inserted into 'rial' is 'RITUAL'.
(Other definitions for ritual that I've seen before include "Ceremony following a prescribed order" , "Custom; ceremony" , "Actions performed as in a religious ceremony" , "Codified ceremony" , "Custom, rite" .)