obstruct, conceal
閼 centers on blocking or hiding something: obstructing passage or concealing from view. The two meanings are closely linked by the idea of preventing access or visibility.
閼 combines 門 (gate) with 於, which likely contributes the sound and may relate to obstructing or blocking at a gate. The exact historical development is uncertain.
A gate 門 with something 於 wedged inside it: the gate is blocked, so passage is obstructed and what lies beyond is concealed.
For ア, imagine a gate that is stuck and won't open, so you let out a frustrated 'Ah!' sound: Ah -> ア, as you try to push through the obstruction.