According to a story on the front page "To support his claim, the USADA (United States Anti-Doping Agency) revealed that the levels of the banned substance, clenbuterol, were indeed minimal and gave no indication of any intent to gain any athletic advantage. Furthermore, the third and most recent sample taken by the USADA was deemed negative and showed no trace of the banned substance. So the fight should take place as scheduled in the main event of the evening." Also, I know it's on a banned list but clenbuterol is hardly a performance enhancer per se. Sure it'll boost your metabolism and enable you to burn off a load more bodyfat but people who use the stuff often find it impedes their cardiovascular capacity. Also it'll give you the shakes something wicked due to stimulating the adrenergic beta 2 receptors. Other common side effects include headaches, cramps, insomnia, sweating, nausea, palpitations and possibly cardiac hypertrophy.
Yes a bodybuilder may well do as part of a cutting stack, but it's hugely unlikely that a boxer would use winstrol ( Stanozolol), because it's very easily detected. In fact it'll show up in urine tests sometimes 4 months after cessation of use. If I remember rightly it was oral winstrol that Ben Johnson was tested positively for.