Browsing some Japanese I found something which I guessed must be new grammar: 「じょうずく」. First, what I do know that seemed related is how to make adverbs. For な-adjectives: simply add に…
じょうず to be good at something
じょうずに to something well
For i-adjectives to make an adverb we loose an ‘い’ and add ‘く’:
はやい (quick) ＞ はやく (quickly)
So, this ‘new grammar’ looked like じょうずに but formed a little as though じょうず was an i-adjective. The answer to the riddle was kanji. My studies told me that the kanji for じょうず was 上手 and hence I read 上手く as じょうずく. However, 上手く is actually read as 「うまく」！
I’d never asked myself what the kanji for うまく was. Though it makes sense now I’d still like to know why this jyukugo has two readings and why there is both a semi i-adjective style adverb and な-adjective style adverb for the same word!