惕 rarely stands alone in modern Chinese - it usually appears in compounds: 警惕 'be vigilant', 惕厉 'cautious and stern', 惕然 'apprehensively'. The 心 (heart) radical signals it describes a state of mind.
忄 is the left-side form of 心 (heart). It marks 惕 as a state of mind - alertness, vigilance, the watchful feeling - and groups with 怕 (fear), 慎 (cautious), 慌 (panic) as part of the wary-emotion family.
易 gives the sound, shifted to tì. The same phonetic feeds 踢 (tī, kick), 锡 (xī, tin) and 赐 (cì, bestow) - a t-/x-/c- sound family branching off the 易 stem.
