Realise, twig (7)
I believe this is a double definition.
'realise' is the first definition.
(I know that catch on is a kind of realise)
'twig' is the second definition.
(synonyms)
(Other definitions for catch on that I've seen before include "Informally, become popular or come to understand" , "Understand - become fashionable" , "twig" , "Get it" .)