18 across says “every third letter in the word for “inability to visualise””.
You'd need to figure out what word this is, and then visualise every third letter of it. Which you can't do if you have it!
late 19th century: coined in German from dys- ‘difficult’ + Greek lexis ‘speech’ (apparently by confusion of Greek legein ‘to speak’ and Latin legere ‘to read’).
all those kinds of imparements are puns. like haveing a lisp. or to stutter. all speech and reading imparements are words that are hard to pronounce or read by the people affected. no idea why. just saying its the rule and no exception.