Nolvadex (tamoxifen citrate) is very comparable to Clomid, behaves in the same manner in all tissues, and is a mixed estrogen agonist/antagonist of the same type as Clomid. The two molecules are also very similar in structure. It is not correct that Nolvadex reduces levels of estrogen: rather, it blocks estrogen from estrogen receptors and, in those tissues where it is an antagonist, causes the receptor to do nothing. Nolvadex is not an anabolic steroid but is used to manage side effects from steroids.
Tamoxifen itself is a prodrug, having relatively little affinity for its target protein, the estrogen receptor. It is metabolized in the liver by the cytochrome P450 isoform CYP2D6 and CYP3A4 into active metabolites such as 4-hydroxytamoxifen (afimoxifene) and N-desmethyl-4-hydroxytamoxifen (endoxifen) which have 30-100 times more affinity with the estrogen receptor than tamoxifen itself. These active metabolites compete with estrogen in the body for binding to the estrogen receptor.