林 (forest) on top originally pictured the thorny chastetree groves of southern China - 楚 first named a particular bramble. From 'thorn,' it became the name of the great southern state of Chu, and the sharp-clarity meaning 'clear, distinct' extended later. Indexed under Kangxi #75 木 via the tree-pair visibly forming this top component.
疋 supplies the sound, drifting from shū to chǔ - a sh-/ch- alternation attested in old Chinese. Same phonetic appears in 楚, 础 (foundation, simplified 础). No semantic role for 疋 here; pure sound.
Filed under radical 木 (mù, #75) by convention. 木 is not a separate component in 楚, so no strokes are highlighted.
