There are a couple of mechanisms whereby tamoxifen might promote fat loss. Aside from the obvious one of elevating test levels, tamoxifen inhibits the activity of lipoprotein lipase (LPL), the enzyme responsible for cleaving glycerol from fatty acids and allowing the fats to enter adipocytes (1). Testosterone and beta agonists contribute to blocking fat uptake by inhibiting LPL as well.
Tamoxifen also increases levels of leptin, one of the so called "anti obesity hormones" (2).
(1) Horm Res 2000;53(1):36-9
Tamoxifen inhibits lipoprotein activity: in vivo and in vitro studies.
Hozumi Y, Kawano M, Hakamata Y, Miyata M, Jordan VC.
(2) Jpn J Clin Oncol 2001 Sep;31(9):424-7
Effects of tamoxifen on the serum leptin level in patients with breast cancer.
Ozet A, Arpaci F, Yilmaz MI, Ayta H, Ozturk B, Komurcu S, Yavuz AA, Tezcan Y, Acikel C.