clarify
闡 centers on making something clear or explicit: clarifying, explaining, or opening up an idea so it is understood.
闡 combines 門 (gate) with 單, which likely contributes the sound and suggests opening or spreading out, giving the idea of opening up meaning to clarify.
Inside a gate 門, the complex shape 單 looks like a person opening a scroll to reveal its contents. Opening the gate to knowledge clarifies what was hidden.
For セン, imagine a sensei opening the gate to clarify a difficult lesson: sensei -> セン, and the gate opens to reveal clear understanding.
clarification; elucidation; explanation