Pressurise baron to go without armour? (9)
I believe the answer is:
blackmail
'pressurise' is the definition.
'baron to go without armour?' is the wordplay.
'baron' becomes 'b'.
'to go without' becomes 'lack'.
'armour?' becomes 'mail' (mail is a kind of armour**).
'b'+'lack'+'mail'='BLACKMAIL'
(Other definitions for blackmail that I've seen before include "Extort illegally" , "Put the screws on" , "Crime of demanding money by threat of exposure" , "Extort money by threats to divulge information" , "Money demanded under threat of revealing unpleasant facts" .)