Cotton on stick (4)
I believe the answer is:
twig
This clue is a double definition.
'cotton on' is the first definition.
(both can mean to realise something)
'stick' is the second definition.
(small stick or branch)
(Other definitions for twig that I've seen before include "bit of wood" , "Small tree shoot" , "Small piece of wood" , "Cotton on - small branch" , "very little from box, perhaps" .)