Unite around one heartless crook that's evil (6)
I believe the answer is:
wicked
'evil' is the definition.
('wicked' is associated in meaning with 'evil')
'unite around one heartless crook' is the wordplay.
'unite' becomes 'wed' (wedding is a kind of uniting).
'around' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'heartless' suggests removing the centre.
'crook' with its centre taken out is 'ck'.
'i'+'ck'='ick'
'wed' going around 'ick' is 'WICKED'.
'that's' is the link.
(Other definitions for wicked that I've seen before include "Sinful" , "Very bad, or very good nowadays" , "Ungodly; excellent (sl.)" , "[SMASHING, COOL, DANDY]" , "Evil or morally wrong" .)