cover, place on top of, west radical variant (no. 146)
襾 centers on covering or placing something on top of another. The radical sense is a variant of the west radical used as a component in other characters.
襾 is a variant form of 西, which originally depicted a bird in a nest or a covered container, later associated with the idea of covering. The exact historical development is uncertain.
The shape looks like a lid placed over a container: the top horizontal line is the lid, and the inner strokes are the container being covered.
For ア, imagine a cover that makes a soft 'ah' sound when you place it on top: ah -> ア, like a satisfied sigh when something is neatly covered.