S.G. Maclean's The Bear Pit (Seeker Series Book 4)

London, 1656. 

Oliver Cromwell is now a king in all but name. Three conspirators, an alliance of Royalists and disillusioned Parliamentarians, plan to assassinate him. 

Seeker, though, is preoccupied by the discovery of the body of a man ravaged by a bear - creatures not seen in the capital since bear baiting was banned. The hunt for the killer leads from Kent's coffee house on Cornhill, to a German clockmaker in Clerkenwell, to the desolate Lambeth marshes where no one should venture at night. 

As the motivation behind the murder becomes clearer, Seeker realises just what - and who - he is up against. The Royalists in exile have sent their greatest fighter; a man who will stop at nothing to ensure the Restoration.