Catch on small branch (4)
I believe the answer is:
twig
This clue is a double definition.
'catch on' is the first definition.
(synonyms)
'small branch' is the second definition.
(I've seen this in another clue)
(Other definitions for twig that I've seen before include "Small part of tree - suddenly understand" , "Slender woody shoot" , "Part of a tree branch" , "A small branch - cop on?" , "Suddenly catch" .)