Trade union rough involves mine in depravity (9)
I believe the answer is:
turpitude
'depravity' is the definition.
(I know that depravity can be written as turpitude)
'trade union rough involves mine' is the wordplay.
'trade union' becomes 'TU' (abbreviation for trade union).
'rough' becomes 'rude' (similar in meaning).
'involves' means one lot of letters goes inside another.
'mine' becomes 'pit' (down the pit).
'rude' enclosing 'pit' is 'rpitude'.
'tu'+'rpitude'='TURPITUDE'
'in' is the link.
(Other definitions for turpitude that I've seen before include "Baseness, depravity" , "Depravity, wickedness" , "corruption" , "Depraved or wicked behaviour" .)