It is a safe thing to use, but I would not expect much of a growth from it. @stevesmi is right - if you want something with really prominent results and yet safe, better go with SARMS.
First off - L-Carnitine will do nothing what so ever for building muscle.
L-carnitine is synthesized in the body from the amino acids lysine and methionine. It transfers long-chain fatty acids into the mitochondria where they may be oxidized to produce energy.
As great as that sounds, supplemental L-carnitine doesn't do a whole lot because your body can efficiently already create it's own. Supplementing with it certainly won't hurt but don't expect miracles. If you do decide to supplement with Carnitine, use the acetyl-l-carnitine version as it is the most bioavailable.
Don't expect miracles. The best supplement you can take for fat loss is cardarine -> http://www.evolutionary.org/cardarine/ Cardarine will directly increase fat loss, endurance and recovery. I would argue it's the best supplement on the market today for fat loss.
It really wont help with muscle building. For that effect you really need to eat more and continue training. If you are looking to add effective supplements for helping to build mass look at creatine, whey protein isolate, HCGenerate.