If it is cracked at the plastic I don´t see why not use. Put some hot glue on the hole, just to prevent some water getting in, or dirt, or your fingers...
When it comes to electricity I don't fuck around I'd get rid of the bulb but first find out where you can recycle it. CFL'S have mercury in them.Happy growing and may all of your harvests be bountiful.
A crack indicates a form of shock.The heart of the voltage regulation is possibly thrown out of values,and the weakest part of the glass tube could be fatigued.They are cheap enough...enough said.
Thanks for the fast replys.. its cracked on the glass part incase i wasent clear and i know theres mercury vapor in there which can be toxic.. yet it still lights up so i wasent sure.