This reading appears almost only in 莞尔 and 莞尔一笑 (to give a slight, gentle smile) — a refined, literary expression. Don't confuse it with the city reading 莞 in Dongguan or the plant reading 莞.
The grass radical 艹 on top. Its original meaning was a marsh sedge; the 'smile' and place-name uses are borrowed sounds, so here the plant radical is historical rather than descriptive.
Below the cap, 完 supplies the sound. Its reading shifts across the three pronunciations of this character (wǎn / guǎn / guān), so the phonetic only loosely fixes the sound.
