Heartless about arresting party workers? Shame! (9)
I believe the answer is:
abashment
'shame' is the definition.
(I've seen this before)
'heartless about arresting party workers?' is the wordplay.
'heartless' means to remove the middle letters.
'arresting' is an insertion indicator.
'party' becomes 'bash' (bash is a kind of party**).
'workers?' becomes 'men' (man is a kind of worker).
'about' with its centre taken out is 'at'.
'bash'+'men'='bashmen'
'at' enclosing 'bashmen' is 'ABASHMENT'.
(Other definitions for abashment that I've seen before include "confusion" , "discomfort" , "shame!" , "embarrassment" .)