I have bought a used MacBook Pro Retina, and after three mounths using it, I think I could do a clean install for my Mac. However, if it has an activation lock will that stop me using my Mac after a clean install as it could ask me for an iCloud account?
Macs do not have Activation Lock.
The only Find My iPhone lock that is present on OS X is the force lock from selecting the device in the Find My iPhone device list and clicking Lock. Simply reinstalling OS X can't lock a Mac unlike iOS Activation Lock.
Note that Find My Mac will register the serial number (or other hardware identifier) for the purposes of recording a single Mac and that reinstalling OS X will not remove this identifier from Apple's servers and may show a "Find My Mac is already in use" message if you attempt to set it up in the future.