Trade union offensive limiting mine corruption (9)
I believe the answer is:
turpitude
'corruption' is the definition.
(I know that corruption can be written as turpitude)
'trade union offensive limiting mine' is the wordplay.
'trade union' becomes 'TU' (abbreviation for trade union).
'offensive' becomes 'rude'.
'limiting' indicates putting letters inside.
'mine' becomes 'pit' (down the pit).
'rude' enclosing 'pit' is 'rpitude'.
'tu'+'rpitude'='TURPITUDE'
(Other definitions for turpitude that I've seen before include "Depraved or wicked behaviour" , "Depravity, wickedness" , "Baseness, depravity" , "corruption" .)