Attitude shown by a heartless posse (4)
I believe the answer is:
pose
'attitude' is the definition.
(I've seen this before)
'a heartless posse' is the wordplay.
'a heartless' suggests removing the centre.
'posse' with its centre taken out is 'POSE'.
'shown by' acts as a link.
(Other definitions for pose that I've seen before include "Models do it for photographic or artistic purposes" , "Model; attitude" , "Pretence" , "front" , "Attitude struck" .)