Shake down old revolutionary that's turned up (6)
I believe the answer is:
extort
'shake down' is the definition.
(thesaurus)
'old revolutionary that's turned up' is the wordplay.
'old' becomes 'ex'.
'revolutionary' becomes 'trot' (abbreviation of Trotskyite).
'that's turned up' says the letters should be written in reverse.
'trot' in reverse letter order is 'tort'.
'ex'+'tort'='EXTORT'
(Other definitions for extort that I've seen before include "To bleed" , "Squeeze" , "Milk" , "Obtain by threats or force" , "Gain using compulsion" .)