Catch on, realise (4)
I believe the answer is:
twig
I believe this clue is a double definition.
'catch on' is the first definition.
(synonyms)
'realise' is the second definition.
(I know that twig is a more specific form of realise)
(Other definitions for twig that I've seen before include "Suddenly realise" , "Small part of a branch - suddenly understand" , "Eventually grasp" , "A little stick" , "Suddenly see; branchlet" .)