Thin branch; realise (4)
I believe the answer is:
twig
I believe this clue is a double definition.
'thin branch' is the first definition.
(I've seen this before)
'realise' is the second definition.
(I've seen this before)
(Other definitions for twig that I've seen before include "Small part of a branch - suddenly understand" , "small piece in plane, say" , "A little stick" , "Suddenly see; branchlet" , "Get the idea" .)