Shake up justice: old fashion is heartless (6)
I believe the answer is:
jostle
'shake' is the definition.
(thesaurus)
'up justice old fashion is heartless' is the wordplay.
'up justice' becomes 'j' (I am not sure about the 'up' bit.).
'old' becomes 'o' (abbreviation).
'fashion' becomes 'style' (synonyms).
'is heartless' suggests removing the centre.
'style' with its centre taken out is 'stle'.
'j'+'o'+'stle'='JOSTLE'
Can you help me to learn more?
(Other definitions for jostle that I've seen before include "Bump and push (for position)" , "Push or bump roughly against someone" , "Hustle" , "Barge" , "Push and shove" .)