Cotton on sticky part of plant (4)
I believe the answer is:
twig
I believe this is a double definition.
'cotton on' is the first definition.
(both can mean to realise something)
'sticky part of plant' is the second definition.
(stick-like part of a tree)
(Other definitions for twig that I've seen before include "bit of stick" , "Tiny end of branch" , "Suddenly realise" , "Cotton on - small branch" , "Suddenly catch" .)