The 5-C compound formed during TCA cycle is :

During TCA cycle, after first decarboxylation of oxalosuccinic acid, $\alpha$-Ketoglutaric acid 5-C compound ( $\alpha$-KGA) is formed. $\alpha$-KGA undergoes further decarboxylation to form a 4 carbon compound i.e., succinyl-CO-A.