Get stick ... (4)
I believe the answer is:
twig
I believe this clue is a double definition.
'get' is the first definition.
(both can mean to understand or realise)
'stick' is the second definition.
(small stick or branch)
(Other definitions for twig that I've seen before include "Informally grasp meaning" , "growth of smaller branches" , "very little from box, perhaps" , "a small piece of gum?" , "Suddenly get" .)