I just encrypted my entire hard drive with truecrypt and was wondering can I still defrag it?
Or will that not work anymore or break something.
Usually, yes you can since the encrypting file system is presenting itself as a traditional file system to Windows and unaware applications.
The larger question is what do you plan to gain with defragmentation since it was conceived when systems were older and slower and fragmented volumes could impact systems performance.
If you're striving to max out your systems performance, defragmentation is the bottom of the list as far as return of investment.